METHODS FOR THE CONTROLLED SYNTHESIS OF LAYERED LITHIUM AND SODIUM TRANSITION METAL OXIDES USING ELECTROCHEMICALLY ASSISTED ION-EXCHANGE

Abstract

Methods for synthesizing layered lithium transition metal oxides from layered sodium transition metal oxides are provided. Also provided are electrodes for lithium-ion batteries that include the layered lithium transition metal oxides. Similarly, methods for the synthesis of layered sodium transition metal oxides from layered lithium transition metal oxides and electrodes for sodium-ion batteries that include the layered sodium transition metal oxides are provided. The methods couple electrochemical intercalation of alkali ions (Li.sup.+ or Na.sup.+) with ion-exchange to overcome the kinetic limitation of ion-exchange in the layered alkali transition metal oxides at low vacancy concentrations.

Claims

1. A method for synthesizing a layered oxide having the formula Li.sub.xNa.sub.yMO.sub.2, where 0<x<1 and 0<y<1 and M is Co, Mn, or Ni, the method comprising: (a) providing a layered cobalt metal oxide having the formula Na.sub.yMO.sub.2, where 0.57≤y≤0.67; (b) conducting a first cation-exchange on the Na.sub.yMO.sub.2, where 0.57≤y≤0.67, in a solution containing dissolved lithium ions to convert the Na.sub.yMO.sub.2, where 0.57≤y≤0.67, into a material comprising discrete phases of Li.sub.0.94MO.sub.2 and Na.sub.yMO.sub.2, where 0.45<y<0.51; (c) conducting an electrochemical intercalation of lithium ions into the material to increase the Li.sub.0.94MO.sub.2 fraction in the material and regenerate Na.sub.yMO.sub.2, where 0.57≤y≤0.67; and (d) conducting an additional cation-exchange on the material in the solution containing dissolved lithium ions material to increase the Li.sub.0.94MO.sub.2 fraction in the material and convert the Na.sub.yMO.sub.2, where 0.57≤y≤0.67, back into Na.sub.yMO.sub.2, where 0.45<y<0.51.

2. The method of claim 1, further comprising repeating steps (c) and (d) two or more times.

3. The method of claim 2, comprising repeating steps (c) and (d) until the Li.sub.0.94MO.sub.2 fraction in the material is at least 90 mol %.

4. The method of claim 1, wherein the solution containing dissolved lithium ions comprises a mixture of dissolved lithium ions and dissolved sodium ions and the dissolved sodium ions are present in excess.

5. The method of claim 1, wherein the solution containing dissolved lithium ions has a lithium ion concentration of 1 mM or lower.

6. The method of claim 1, wherein the solution containing dissolved lithium ions has a lithium ion concentration 0.2 mM or lower.

7. The method of claim 1, wherein M is Co.

8. The method of claim 1, wherein M is Mn.

9. The method of claim 1, wherein M is Ni.

10. An electrode comprising a material comprising discrete Li.sub.0.94MO.sub.2 and Na.sub.yMO.sub.2 phases, where 0.45<y<0.51, wherein the phase fraction of Li.sub.0.94MO.sub.2 in the material is at least 90 mol. %.

11. The electrode of claim 10, wherein the discrete Li.sub.0.94MO.sub.2 and Na.sub.yMO.sub.2 phases are disposed on an electrically conductive substrate.

12. A method for synthesizing a layered oxide having the structure Na.sub.yMO.sub.2, where 0.45<y<0.51 and M is Co, Mn, or Ni, the method comprising: providing a layered cobalt metal oxide having the structure LiMO.sub.2; conducting an electrochemical deintercalation of lithium ions from the material to convert the LiMO.sub.2 into Li.sub.0.4MO.sub.2; and conducting a cation-exchange on the Li.sub.0.4MO.sub.2 in a solution containing dissolved sodium ions to convert the Li.sub.0.4MO.sub.2 into a material comprising Na.sub.yMO.sub.2 phases, where 0.45<y<0.51, wherein the phase fraction of Na.sub.yMO.sub.2 in the material is at least 0.98 mol. %.

13. The method of claim 12, wherein M is Co.

14. The method of claim 12, wherein M is Mn.

15. The method of claim 12, wherein M is Ni.

16. An electrode comprising a material comprising Na.sub.yMO.sub.2 phases, where 0.45<y<0.51, wherein the phase fraction of Na.sub.yMO.sub.2 in the material is at least 0.98 mol. %.

17. The electrode of claim 16, wherein the Na.sub.yMO.sub.2 phases are disposed on an electrically conductive substrate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Illustrative embodiments of the invention will hereafter be described with reference to the accompanying drawings.

[0011] FIGS. 1A-1E show phase separation and two-phase equilibrium between Li.sub.0.94CoO.sub.2 and Na.sub.0.48CoO.sub.2. FIG. 1A shows galvanostatic curves of P3-Na.sub.yCoO.sub.2 in the range of 0.37<y<0.92 and Li.sub.xCoO.sub.2 in the range of 0.37<x<1.0 at C/80. FIG. 1B shows open-circuit voltage (OCV) curves of P3-Na.sub.yCoO.sub.2 (y=0.37, 0.47, 0.57, 0.67, 0.77, 0.87) during ion exchange. Inset: enlarged OCV curve of Na.sub.0.67CoO.sub.2 showing the “pseudo-charging” behavior with four characteristic plateaus. FIG. 1C shows synchrotron X-ray diffraction patterns of Na.sub.yCoO.sub.2 (y=0.37, 0.57, 0.67, 0.87) before and after 24 h ion exchange. Gray dash lines indicate the (003) peak positions of equilibrated phases Na.sub.0.48CoO.sub.2 (left) and Li.sub.0.94CoO.sub.2 (right). Na.sub.0.57CoO.sub.2 and Na.sub.0.67CoO.sub.2 which showed phase equilibrium behaviors are highlighted. FIG. 1D shows chemical compositions of Li.sub.aNa.sub.bCoO.sub.2. Black dots and white dots are the Li contents and Na contents measured after 24 h and 15 d (labeled by the arrows) ion exchange, respectively, for Na.sub.yCoO.sub.2 (y=0.37, 0.47, 0.57, 0.67, 0.72, 0.77, 0.87). The dash lines are predicted Li and Na contents based on phase equilibrium. FIG. 1E shows Rietveld refinements of patterns in FIG. 1C: Na.sub.0.67CoO.sub.2 (top) after 24 h ion exchange with 39.7% Li.sub.0.94CoO.sub.2 and 60.3% Na.sub.0.48CoO.sub.2 and Na.sub.0.57CoO.sub.2 (bottom) after 24 h ion exchange with 17.9% Li.sub.0.94CoO.sub.2 and 82.1% Na.sub.0.48CoO.sub.2.

[0012] FIGS. 2A-2F show revealing structural evolution during Li ion exchange with Na.sub.0.67CoO.sub.2. FIG. 2A shows a HAADF-STEM image, EDS maps, and Li EELS map of Na.sub.0.67CoO.sub.2 after reaching equilibrium. Na locates in the center and Li locates at the edges and corners of the particle. Scale bar, 30 nm. FIG. 2B shows synchrotron X-ray diffraction patterns of different intermediate states. Gray dash lines indicate the (003) peak positions of equilibrated phases Na.sub.0.48CoO.sub.2 (left) and Li.sub.0.94CoO.sub.2 (right). FIG. 2C shows Lithium contents (measured (dots) and calculated (line) based on evolution equation) as a function of Na phase during ion exchange. FIG. 2D shows OCV curves of Na.sub.0.67CoO.sub.2 ion exchanging in ACN solutions with different Li—Na ratios. The curve of 1 M Li is overlapping with the curve of 1-1 Li—Na. FIG. 2E shows in-situ synchrotron XRD patterns of the Na (003) peak and Li (003) peak during Na.sub.0.67CoO.sub.2 ion exchange in 1 M Li ACN solution. The nucleation and left-shift of the Li.sub.0.94CoO.sub.2 peak starting from 1000 s accompanied by Na (003) peak left-shift were clearly revealed. FIG. 2F is a schematic showing both the surface reaction-limited and diffusion-limited exchange pathways at low Li and high Li ratios, respectively. Na.sub.0.64 is one example Na phase of a surface reaction-limited ion exchange pathway.

[0013] FIGS. 3A-3E show reverse conversion from Li.sub.xCoO.sub.2 to Na.sub.yCoO.sub.2. FIG. 3A shows electrochemical curves of commercial LiCoO.sub.2 after deintercalation of 40%, 50%, and 60% capacities in 1 M Na solution at C/10 with additional 41 h, 40 h and, 39 h soaking, respectively. FIGS. 3B, 3C show XRD patterns (FIG. 3B) and chemical compositions (FIG. 3C) of Li.sub.0.40CoO.sub.2, Li.sub.0.50CoO.sub.2, and Li.sub.0.60CoO.sub.2 after reaching equilibrium in 1 M Na solution. The dash lines are predicted Li and Na contents based on phase equilibrium. FIG. 3D shows an atomic resolution HAADF-STEM image of fully converted Na.sub.0.48CoO.sub.2 along [010] zone axis and the signal profile from the dashed area. Scale bar, 1 nm. FIG. 3E shows a SEM image and EDS maps of fully converted Na.sub.0.48CoO.sub.2. Scale bar, 30 μm. FIG. 3F shows a HAADF-STEM image and EDS maps of fully converted Na.sub.0.48CoO.sub.2. Scale bar, 50 nm.

[0014] FIGS. 4A-4E show full conversion from Na.sub.yCoO.sub.2 to Li.sub.xCoO.sub.2 by electrochemical assisted ion exchange. FIG. 4A shows electrochemical curves of the electrochemical assisted ion exchange process. Once the intercalation current stops, the OCV goes back to equilibrium voltage through the Na phase change. FIG. 4B shows Lithium contents as a function of Na phase during electrochemical assisted ion exchange process. Measured (solid balls), calculated (empty circles) based on voltages and capacities in FIG. 4A, and predicted (gray dash line) based on the equilibrium equation and evolution equation are shown. FIG. 4C shows chemical compositions at different progress points shown in FIG. 4B. FIG. 4D shows XRD patterns of original P3-Na.sub.0.67CoO.sub.2 and fully converted Li.sub.0.90Na.sub.0.02CoO.sub.2. FIG. 4E shows an atomic resolution HAADF-STEM image of fully converted Li.sub.0.90Na.sub.0.02CoO.sub.2 along [010] zone axis and the signal profile from the dashed area. Scale bar, 1 nm.

[0015] FIG. 5 shows a phase diagram of Li and Na interchange in layered cobalt oxides. Two-phase equilibrium between Li.sub.0.94CoO.sub.2 and Na.sub.0.48CoO.sub.2 divides the whole cation range into two different regions. In the region above the equilibrium line, dual ion exchange between Li.sub.xCoO.sub.2 and Na.sub.yCoO.sub.2 (y=x) or Li.sub.xCoO.sub.2 and Li.sub.aNa.sub.bCoO.sub.2 (equilibrium composition) can be achieved. In the region below the equilibrium line, only Na.sub.yCoO.sub.2 exchanging with Li can be approached due to the strong structural Li preference. The kinetic barrier in this single exchange region is extremely large when the vacancy level is inadequate (A>0.72, A=a+b in Li.sub.aNa.sub.bCoO.sub.2). The electrochemical assisted ion exchange pathway is highlighted with black arrows. The thickness of the shaded arrows indicates the driving force of the targeted ion in solutions (e.g. Na.sub.yCoO.sub.2 exchange with 1-1000 Li—Na and 1 M Li are shown by thin and thick arrows, respectively.).

[0016] FIGS. 6A-6D show phase separation and two-phase equilibrium Li.sub.0.94CoO.sub.2—Na.sub.0.46CoO.sub.2 exist in P2-Na.sub.yCoO.sub.2 exchange with 1-1000 Li—Na. FIG. 6A shows a Galvanostatic curve of P2-Na.sub.yCoO.sub.2 in the range of 0.37<y<0.87. FIG. 6B shows open-circuit voltage (OCV) curves of P2-Na.sub.yCoO.sub.2 (y=0.37, 0.47, 0.57, 0.67) during ion exchange. FIG. 6C shows chemical compositions of P2-Na.sub.yCoO.sub.2 (y=0.37, 0.47, 0.57, 0.67) after 24 hours ion exchange. Black dots and white dots are the Li contents and Na contents measured by ICP-MS. The dash lines are predicted Li and Na contents based on phase equilibrium. FIG. 6D shows XRD of P2-Na.sub.0.67CoO.sub.2 after 24 hours ion exchange. The separated Li phase located at 18.54° (2θ) is a Li full phase. Due to the same consideration as for the P3 case, it is assigned as Li.sub.0.94CoO.sub.2.

[0017] FIGS. 7A-7C show the established electrochemical assisted ion exchange can be conducted in 1-10000 Li—Na ACN. FIG. 7A shows electrochemical curves during the whole process. The OCV curves still show the four intermediate plateaus. FIGS. 7B, 7C show the electrochemical intercalation curves for 1.sup.st inter (FIG. 7B, C/10) and 2.sup.nd inter (FIG. 7C, C/40) shown in FIG. 7A. Four intermediate plateaus are also visible in 1-10000 Li—Na. The right panel shows the absolute current during intercalation.

DETAILED DESCRIPTION

[0018] Methods for synthesizing layered lithium transition metal oxides from layered sodium transition metal oxides are provided. Also provided are electrodes for lithium-ion batteries that include the layered lithium transition metal oxides. Similarly, methods for the synthesis of layered sodium transition metal oxides from layered lithium transition metal oxides and electrodes for sodium-ion batteries that include the layered sodium transition metal oxides are provided. The methods couple electrochemical intercalation of alkali ions (Lit or Nat) with ion-exchange to overcome the kinetic limitations of ion-exchange in layered alkali transition metal oxides having low vacancy concentrations.

[0019] One aspect of the invention provides a method of forming layered Li.sub.0.94MO.sub.2 from Na.sub.yMO.sub.2, where 0.57≤y≤0.67 and M represents cobalt (Co), manganese (Mn), or nickel (Ni). The development of this method may be attributed, at least in part, to two discoveries by the inventors. First, the inventors discovered that a two-phase equilibrium exists between Li.sub.0.94MO.sub.2 and Na.sub.yMO.sub.2, where 0.45<y<0.51, for the exchange of sodium ions with lithium ions in the starting Na.sub.yMO.sub.2, where 0.57≤y≤0.67. And, second, the inventors discovered that, while the Li.sub.0.94MO.sub.2 forms via the ion-exchange once the Li phase separates from the Na host phase, the Na phase goes through a series phase changes beginning with the Na.sub.yMO.sub.2 phase where 0.57≤y≤0.67, to the final equilibrated Na.sub.yMO.sub.2 phase, where 0.45<y<0.51, during the ion-exchange.

[0020] By coupling ion-exchange with electrochemical ion intercalation and controlling the phase equilibrium and vacancies during the ion-exchange process, the starting sodium transition metal oxide can be substantially completely transformed into the layered lithium transition metal oxide or a mixed solution of layered lithium and sodium transition metal oxides (i.e., Li.sub.xNa.sub.yMO.sub.2, where 0<x<1 and 0<y<0 and M is Co, Mn, or Ni) can be formed. The exchange of sodium ions with lithium ions in the layered transition metal oxides is a fully reversable process. This controllable and reversible electrochemically assisted ion-exchange process enables the synthesis of meta-stable layered lithium transition metal oxides, layered sodium transition metal oxides, and mixed layered lithium and sodium transition metal oxides that cannot be synthesized directly by more conventional methods, such as solid-state synthesis. As such, the present methods enlarge the layered oxide library for electrodes for both lithium-ion batteries and sodium-ion batteries.

[0021] One embodiment of a method for synthesizing a layered oxide having the formula Li.sub.xNa.sub.yMO.sub.2, where 0<x<1 and 0<y<1 and M is Co, Mn, or Ni, includes the steps of: (a) providing a layered cobalt transition metal oxide having the formula Na.sub.yMO.sub.2, where 0.57≤y≤0.67; (b) conducting a first cation-exchange on the Na.sub.yMO.sub.2, where 0.57≤y≤0.67, in a solution containing dissolved lithium ions to convert the Na.sub.yMO.sub.2, where 0.57≤y≤0.67, into a material comprising discrete phases of Li.sub.0.94MO.sub.2 and Na.sub.yMO.sub.2, where 0.45<y<0.51; (c) conducting an electrochemical intercalation of lithium ions into the material to increase the Li.sub.0.94MO.sub.2 fraction in the material and regenerate Na.sub.yMO.sub.2, where 0.57≤y≤0.67; and (d) conducting an additional cation-exchange on the material in the solution containing dissolved lithium ions to further increase the Li.sub.0.94MO.sub.2 fraction in the material and convert the Na.sub.yMO.sub.2, where 0.57≤y≤0.67, back into Na.sub.yMO.sub.2, where 0.45<y<0.51. Steps (c) and (d) may be repeated multiple times to increase the phase fraction of the Li.sub.0.94MO.sub.2 in the material until a desired phase fraction is achieved. By way of illustration, steps (c) and (d) can be cycled until substantially complete ion-exchange is achieved, where a substantially complete ion exchange results in a phase fraction of Li.sub.0.94MO.sub.2 of at least 90 mol. % in the final material. This includes embodiments in which the phase fraction of Li.sub.0.94MO.sub.2 in the final material is at least 95 mol. % and at least 98 mol. %.

[0022] It is advantageous to use a thermodynamically stable phase of the Na.sub.yMO.sub.2, where 0.57≤y≤0.67, as the starting material for the Li.sub.0.94MO.sub.2 synthesis. For example, P2-Na.sub.0.67MO.sub.2 or P3-Na.sub.0.67MO.sub.2 can be used. However, other stable or meta-stable layered sodium transition metal oxides can also be used.

[0023] The first sodium-lithium cation exchange of the method is carried out on the starting Na.sub.yMO.sub.2, where 0.57≤y≤0.67, in a solution comprising dissolved lithium ions (Lit). The cation-exchange may be carried out at room temperature (23° C.) or near room temperature (for example, at temperatures in the range from 20° C. to 30° C.). However, higher temperatures can be used. Due to kinetic limitations on the cation-exchange, the exchange does not go to completion. Instead, the result of the initial cation-exchange is a two-phase material comprising Li.sub.0.94MO.sub.2 and Na.sub.yMO.sub.2, where 0.45<y<0.51. In order to increase the phase fraction of Li.sub.0.94MO.sub.2 in the material, the ion-exchange is followed by an electrochemical intercalation of lithium ions into the material. This is accomplished by applying a potential (an “intercalation potential”) across the material. Under the influence of this potential, lithium ions in the solution migrate into the layered structure of the alkali transition metal oxides. As a result, the phase fraction of Li.sub.0.94MO.sub.2 in the material is increased and the Na.sub.yMO.sub.2, 0.45<y<0.51, phase is converted back into Na.sub.yMO.sub.2, 0.57≤y≤0.67. This material can then undergo an additional cation-exchange step to further increase the phase fraction of Li.sub.0.94MO.sub.2 in the material, while regenerating the equilibrium Na.sub.yMO.sub.2 phase, where 0.45<y<0.51.

[0024] Notably, because the layered oxides are highly selective for lithium ions over sodium ions, the electrochemical intercalation assisted ion-exchange can be carried out in a solution in which sodium ions are in excess over lithium ions. In fact, dilute solutions in which the lithium ion concentration is lower than 1 millimolar (mM), lower than 0.5 mM, or lower than 0.2 mM (e.g., in the range from about 0.1 mM to 1 mM) can be used. However, the methods can also be carried out in higher concentration solutions and in solutions in which the lithium ions are in excess.

[0025] Another aspect of the invention provides methods for the synthesis of a layered oxide having the formula Na.sub.yMO.sub.2, where 0.45<y<0.51 and M is Co, Mn, or Ni, from the corresponding layered LiMO.sub.2. One embodiment of such a method includes the steps of: providing a layered lithium transition metal oxide having the having the formula LiMO.sub.2; conducting an electrochemical deintercalation of lithium ions from the material to convert the LiMO.sub.2 into Li.sub.0.4MO.sub.2; and conducting a cation-exchange on the Li.sub.0.4MO.sub.2 in a solution containing dissolved sodium ions to convert the Li.sub.0.4MO.sub.2 into a material comprising Na.sub.yMO.sub.2 phases, where 0.45<y<0.51. During the electrochemical deintercalation, a potential is applied across the LiMO.sub.2 to force lithium ions out of the material. The subsequent cation exchange between sodium and lithium ions can completely (100%) or substantially completely (≥90%, 95%, or 98%) convert the LiMO.sub.2 into Na.sub.yMO.sub.2, where 0.45<y<0.51.

[0026] The lithium phase-pure or lithium phase-rich layered transition metal oxides and the sodium phase-pure or sodium phase-rich layered transition metal oxides can be used as the active materials in electrodes for lithium-ion batteries and sodium-ion batteries, respectively. The layered transition metal oxides may be supported on an electrically conductive support substrate, such as a metal substrate, and/or may be mixed with an electrically conductive powder, such as a metal or carbon powder, to form the electrodes. Optionally, a binder may be used to enhance the mechanical integrity of the electrodes.

[0027] One embodiment of a lithium-ion battery includes: a battery compartment; a cathode comprising a material comprising discrete Li.sub.0.94MO.sub.2 and Na.sub.yMO.sub.2 phases, where 0.45<y<0.51, wherein the phase fraction of the Li.sub.0.94MO.sub.2 in the material is at least 90 mol. %; an anode in electrical communication with the cathode; an electrically conductive wire connecting the anode to the cathode; and a lithium ion-conductive electrolyte disposed between the anode and the cathode. Typically, the lithium-ion battery will also include a separator, such as an ion-permeable membrane, in the electrolyte between the anode and the cathode in order to physically separate the anode from the cathode.

[0028] One embodiment of a sodium-ion battery includes: a battery compartment; a cathode comprising a material comprising a Na.sub.yMO.sub.2 phase, where 0.45<y<0.51, wherein the phase fraction of the Na.sub.yMO.sub.2 in the material is at least 0.98 mol. %; an anode in electrical communication with the cathode; an electrically conductive wire connecting the anode to the cathode; and a sodium ion-conductive electrolyte disposed between the anode and the cathode. Typically, the sodium-ion battery will also include a separator, such as an ion-permeable membrane, in the electrolyte between the anode and the cathode in order to physically separate the anode from the cathode.

Example

[0029] The Example provides a detailed discussion of predictive ion exchange pathways and reveals an ion exchange mechanism for Li and Na in layered oxides using cobalt oxides as models. Counterintuitively, using Li ions at extremely low molar ratios (e.g. 1-1000 molar ratio Li—Na) and small excess (e.g. 18% excess of Li to target amount), near equilibrium exchange with Na.sub.yCoO.sub.2 can be achieved by taking advantage of structural Li preference. Instead of forming Li.sub.xCoO.sub.2 with the same cation content (x=y), the structure nucleates a Li.sub.0.94CoO.sub.2 phase that has the smallest potential difference with the Na phase, which drives the Na phase change to form the equilibrium between Na.sub.0.48CoO.sub.2 and Li.sub.0.94CoO.sub.2. The phase separation and equilibrium behaviors allow for the prediction of not only the final compositions and phases, but also the intermediate states to map out the kinetic pathways. The phase separation behavior was also captured at far from equilibrium conditions with high Li concentrations and large Li excesses. This Example also demonstrates that Li.sub.0.94CoO.sub.2 nucleation is a critical step to initiate the ion exchange, following which the reaction proceeds with either a diffusion-limited (high Li ratio) or a surface reaction-limited (low Li ratio) mechanism. Additionally, a large kinetic energy barrier at low vacancy levels is also identified, which defines the accessible and inaccessible ion exchange pathways. Guided by the understanding in vacancy-dependent ion preference and diffusion barriers, Na.sub.yCoO.sub.2 (˜98% Na purity) conversion from the parent Li.sub.xCoO.sub.2 was identified for the first time and Na.sub.yCoO.sub.2 conversion to Li.sub.0.94CoO.sub.2 (˜98% Li purity from 1-1000 molar ratio Li—Na) via electrochemical assisted ion exchange was also identified, with the latter being of significant importance for Li extraction.

[0030] Na.sub.0.48CoO.sub.2 and Li.sub.0.94CoO.sub.2 Phase Equilibrium

[0031] Platelet-like P3-Na.sub.0.67CoO.sub.2 particles were used as model materials to systematically explore the ion exchange process. P3-Na.sub.0.67CoO.sub.2 particles have a size around 100-500 nm with a thickness less than 100 nm. FIG. 1A shows the galvanostatic curves of O3-Li.sub.xCoO.sub.2 and P3-Na.sub.yCoO.sub.2 at slow kinetics (C/80) which represent mostly their thermodynamic differences. O3-Li.sub.xCoO.sub.2 and P3-Na.sub.yCoO.sub.2 have distinct phase transformations with respect to vacancy change. Importantly, the significant voltage differences between Li.sub.xCoO.sub.2 and Na.sub.yCoO.sub.2 (e.g. ˜1.1V at x=y=0.67) shown on the galvanostatic curves indicates the preference for Li. Such structural Li preference is a function of the vacancy level wherein the Li preference increases as the vacancy level decreases.

[0032] To control the ion exchange and limit possible kinetic pathways, the ion exchange reaction was designed near equilibrium using a low Li ratio (1-1000 Li—Na, 1 mM Li and 1 M Na in acetonitrile (ACN) solution, unless otherwise specified). Na.sub.yCoO.sub.2 with varying y (y=0.37, 0.47, 0.57, 0.67, 0.77, 0.87) were prepared to investigate the ion exchange at different structural Li preferences (as marked by DV=V.sub.LixCoO2−V.sub.NayCoO2). The open-circuit voltage (OCV) of P3-Na.sub.yCoO.sub.2 during ion exchange was monitored. Interestingly, the OCV curve of Na.sub.0.67CoO.sub.2 has a shape similar to the galvanostatic curve at the range between y=0.67 and y=0.48 but with an additional final plateau at the voltage of 3.45V (FIG. 1B). Four intermediate plateaus (I, II, III, and IV) on the galvanostatic charging curve also appear on the OCV curve of Na.sub.0.67CoO.sub.2 (inset of FIG. 1B), which indicates the occurrence of similar phase transformations. The differential capacity curve of P3-Na.sub.0.67CoO.sub.2 has four peaks in the voltage range of 2.8V-3.6V, which correspond to the four plateaus shown in the galvanostatic charging curve. This “pseudo-charging” behavior (potential changing like charging but without redox reactions) suggests that ion exchange with Li can induce the structure change of the starting Na.sub.yCoO.sub.2. The “pseudo-charging” is also observed at Na content (y) of 0.57. However, for other sodium contents (y=0.37, 0.47, 0.77, and 0.87), no “pseudo-charging” behavior was observed.

[0033] Synchrotron X-ray diffraction (XRD) was conducted to identify the Li and Na phases after ion exchange. For parent Na.sub.yCoO.sub.2 before ion exchange, the (003) peaks gradually left-shifted as the sodium content y decreased, corresponding to the expansion of interlayer distance (FIG. 1C). After ion exchange, rather than forming the intermediate phase observed at the elevated temperature, a new Li phase appeared for Na.sub.0.57CoO.sub.2 and Na.sub.0.67CoO.sub.2 which showed “pseudo-charging” behaviors. The (003) peak of the new Li phase was at the same position and assigned to Li.sub.0.94CoO.sub.2. Accompanied by the Li phase appearance, the Na (003) peaks of the Na.sub.0.57CoO.sub.2 and Na.sub.0.67CoO.sub.2 left-shifted to the position of the Na.sub.0.48CoO.sub.2 phase (see Methods for Li.sub.0.94CoO.sub.2 and Na.sub.0.48CoO.sub.2 phase assignment). Varying the ion exchange time did not affect the Li contents for Na.sub.0.67CoO.sub.2, which illustrates the equilibrium for Na.sub.0.67CoO.sub.2 had been established within 12 hours. This, and the continuation of the final plateau (FIG. 1B), indicate the ion exchange process already reached a steady state and the formed Li.sub.0.94CoO.sub.2 phase and Na.sub.0.48CoO.sub.2 phase were in equilibrium. However, for Na.sub.0.37CoO.sub.2 and Na.sub.0.87CoO.sub.2, no obvious Li phase was observed after 24 h ion exchange based on synchrotron XRD characterization.

[0034] If two-phase equilibrium between Li.sub.0.94CoO.sub.2 and Na.sub.0.48CoO.sub.2 exists in the ion exchange process, the Li and Na contents a and b can be calculated in the structure Li.sub.aNa.sub.bCoO.sub.2 based on the vacancy level (see equations in Methods). The chemical compositions are Li.sub.0.47Na.sub.0.25CoO.sub.2, Li.sub.0.39Na.sub.0.28CoO.sub.2, and Li.sub.0.21Na.sub.0.36CoO.sub.2 after ion exchange for the starting materials Na.sub.0.72CoO.sub.2, Na.sub.0.67CoO.sub.2, and Na.sub.0.57CoO.sub.2. The excellent agreement between the measured chemical compositions and predicted chemical compositions based on the equilibrium equation confirms the two-phase equilibrium between Li.sub.0.94CoO.sub.2 and Na.sub.0.48CoO.sub.2 (FIG. 1D), which is also proved by Rietveld refinement results (FIG. 1E). However, based on the two-phase equilibrium, Na.sub.0.77CoO.sub.2 and Na.sub.0.87CoO.sub.2 should convert to Li.sub.0.59Na.sub.0.18CoO.sub.2 and Li.sub.0.80Na.sub.0.07CoO.sub.2, respectively. The experiment results showed that the final compositions (Li.sub.0.21Na.sub.0.56CoO.sub.2 and Li.sub.0.07Na.sub.0.80CoO.sub.2, respectively) had much less Li than the predicted value, indicating kinetic limitations. Such kinetic barrier is so high that even after 15 days of ion exchange, the Li content a was still 0.17 instead of 0.90 for Na.sub.0.92CoO.sub.2, and even when the exchange solution was changed to 1 M Li, the ion exchange could not go to completion. A large kinetic barrier may be partially due to the vastly different diffusion coefficients (approximately two orders of magnitude) from the ˜30% to ˜10% vacancy level. The following section demonstrates that this kinetic limitation can be overcome via an electrochemical assisted ion exchange process.

[0035] Based on the final compositions and phases, the ion exchange process was divided into three different regions. First, in the range of 0.48≤a+b≤0.72 (FIG. 1D), phase equilibrium was established between Li.sub.0.94CoO.sub.2 and Na.sub.0.48CoO.sub.2. Second, with 0.72<a+b≤0.94 (FIG. 1D), such as starting from Na.sub.0.77CoO.sub.2 and Na.sub.0.87CoO.sub.2, even though two-phase equilibrium was predicted, the ion exchange could not complete due to large kinetic barriers at low vacancy levels. Third, for 0.37≤a+b≤0.48 (FIG. 1D), no “pseudo-charging” behavior was observed and Na.sub.yCoO.sub.2 was the stable phase without ion exchange. As indicated by the potentials in FIG. 1A, with y in Na.sub.yCoO.sub.2 decreasing to 0.48, the structural preference for Li decreased significantly (DV decreases from 1.1V to 0.5V). With the low starting Li ratio (1-1000 Li—Na), the driving force was insufficient to initiate the ion exchange. By using a higher Li ratio (1 M Li), the ion exchange of Na.sub.0.37CoO.sub.2 was complete, confirming the limitation was from the thermodynamic driving force. However, such ion exchange does not follow the two-phase equilibrium route between Li.sub.0.94CoO.sub.2 and Na.sub.0.48CoO.sub.2, since the cation content is out of the equilibrium range.

[0036] This phase separation and equilibrium phenomenon accompanied by the “pseudo-charging” behavior is not unique to the P3-Na.sub.yCoO.sub.2 system. Similar phenomena were observed for P2-Na.sub.yCoO.sub.2 exchanged with Li in 1-1000 Li—Na CAN (FIG. 6). Based on the same characterization, the equilibrated Li phase and Na phase in the P2-Na.sub.yCoO.sub.2 system were assigned as O2-Li.sub.0.94CoO.sub.2 and P2-Na.sub.0.46CoO.sub.2, respectively.

[0037] Resolving Ion Exchange Pathways

[0038] Before resolving ion exchange pathways, it was first necessary to understand the phase equilibrium between Li.sub.0.94CoO.sub.2 and Na.sub.0.48CoO.sub.2. From the galvanostatic curves, Li.sub.0.94CoO.sub.2 and Na.sub.0.48CoO.sub.2 have similar potentials that allow the structure to establish equilibrium. In contrast, direct conversion of Na.sub.0.67CoO.sub.2 to Li.sub.0.67CoO.sub.2 will cause a large potential difference at the reaction interface which could lead to structural instability. Moreover, the phase equilibrium between Li.sub.0.94CoO.sub.2 and Na.sub.0.48CoO.sub.2 indicates that charge redistribution between Co.sup.3+/Co.sup.4+ must occur during the phase change. The in-plane electron resistivity is much lower than that of out-of-plane and across-particle electron resistivities. Therefore, in-layer intra-particle phase separation would be the most feasible pathway to establish the phase equilibrium. Scanning transmission electron microscopy energy-dispersive x-ray spectroscopy (STEM EDX) and electron energy loss spectroscopy (EELS) were then performed to resolve the Na and Li distribution, respectively. The results proved the in-layer intra-particle phase separation with Na mainly in the center and Li at the corners and edges in a single hexagonal-like particle (FIG. 2A). Despite the fact that Li and Na are commonly thought to not coexist in one layer, this nonuniform distribution of Li and Na in the basal plane indicates that the ion exchange process does not follow the slab-by-slab exchange route.

[0039] To gain the full picture of ion exchange pathways, several intermediate states during ion exchange were characterized by synchrotron XRD. The ion exchange process was quenched at the I, II, and IV plateaus, and denoted as the state I, II, and IV respectively. At state I, the structure directly formed Li.sub.0.94CoO.sub.2 (FIG. 2B) instead of Li.sub.0.67CoO.sub.2 as proposed in the slab gliding model. (Tournadre, F. et al., Journal of Solid State Chemistry 177, 2803-2809 (2004).) The formation of Li.sub.0.94CoO.sub.2 is favorable, since it has the smallest potential difference to Na.sub.yCoO.sub.2 (y>0.48). The observed phase transformation indicates that the ion exchange started with Li.sub.0.94CoO.sub.2 nucleation, and then the Na phase changed from Na.sub.0.67CoO.sub.2 to the final equilibrated phase Na.sub.0.48CoO.sub.2 as the Li phase grew. The persistence of the Li.sub.0.94CoO.sub.2 phase during the entire ion exchange allowed for the determination of the chemical composition and phase evolution of Li.sub.aNa.sub.bCoO.sub.2 based on the evolution equation (Methods). The chemical compositions calculated based on the evolution equation, inductively coupled plasma mass spectrometry (ICP-MS) measurement, and Rietveld refinement all showed good agreement, supporting the proposed evolution pathway (FIG. 2C).

[0040] To understand whether the proposed phase separation process is a general evolution pathway, the ion exchange process was studied at different Li—Na ratios. All the OCV curves showed “pseudo-charging” behavior in 1-1000, 1-100, 1-10, 1-1 (molar ratio Li—Na, Na concentration was fixed as 1 M), and 1 M Li acetonitrile solution, but with different plateau numbers and final plateau potentials. The difference in final plateau potentials can be understood from the Nernst shift of the potential of the Li.sub.0.94CoO.sub.2 phase in solutions with different Li concentrations (FIG. 2D). The four intermediate plateaus during Na phase transformation were still visible in 1-100 and 1-10 cases. For 1-1 and 1 M Li, the fast exchange kinetics makes capturing intermediate steps challenging. But the appearance of the first plateau was observed. In situ synchrotron XRD was then performed for the case of 1 M Li. The emergence of the Li.sub.0.94CoO.sub.2 (003) peak at 1225 s 1375 s (FIG. 2E) demonstrated again that the exchange process initiates via Li.sub.0.94CoO.sub.2 nucleation. However, in this condition, both Li and Na phases changed after Li.sub.0.94CoO.sub.2 nucleation, as shown by the left-shift of both Li and Na peaks (FIG. 2E), and then finally a single Li.sub.0.67CoO.sub.2 phase was established with the disappearance of the Na phase.

[0041] This result points to two ion exchange routes following Li.sub.0.94CoO.sub.2 nucleation. When the solution Li ratio is low (e.g. 1-1000 Li—Na), Li exchange with surface Na is the rate-limiting step (surface reaction-limited). Structural Na can diffuse to fill up the vacancy formed from Li.sub.0.94CoO.sub.2 phase nucleation and growth. Therefore, the Na phase change and a persistent Li.sub.0.94CoO.sub.2 phase in the whole exchange process (FIG. 2F) were only observed. When the solution Li ratio is high (e.g. 1 M Li), Li can quickly exchange with surface Na and nucleate a Li.sub.0.94CoO.sub.2 phase. The fast nucleation and growth of the Li.sub.0.94CoO.sub.2 phase leaves nearby domains with much higher vacancy levels. In this case, bulk Na diffusion alone cannot catch up with the ion exchange rate (diffusion-limited) and the diffusion of both Li and Na occurs to avoid forming unstable interfaces with large potential differences. Additionally, high vacancy Na phases (Na.sub.yCoO.sub.2, y≤0.48) can directly exchange with solution Li, skipping the Li.sub.0.94CoO.sub.2 nucleation when the solution Li ratio is high (e.g. 1 M Li). Hence, in the P3-Na.sub.yCoO.sub.2 in-situ synchrotron XRD, the left-shift of the Na peak caused by Li.sub.0.94CoO.sub.2 nucleation and growth was observed. Then the Li peak left-shifted to Li.sub.0.67CoO.sub.2 due to the merging of Li.sub.0.94CoO.sub.2 with high vacancy Li domains (formed from the direct conversion of high vacancy Na phases). The diffusion-limited ion exchange was also observed for P2-Na.sub.yCoO.sub.2 exchanging with 1 M Li. Due to the large particle size, both Li.sub.0.94CoO.sub.2 and high vacancy Li.sub.xCoO.sub.2 (x≤0.46) were observed without merging accompanied by a high vacancy Na.sub.yCoO.sub.2 (y≤0.46) phase.

[0042] Pure Na.sub.yCoO.sub.2 from Li.sub.xCoO.sub.2 Via Reversed Ion Exchange

[0043] Despite successful ion exchange to make Li cathodes, the reversed ion exchange with Na replacing the structural Li to achieve pure Na.sub.yCoO.sub.2 has not been reported before. Starting with a full Li structure without any vacancy only allows less than 1% of exchange at elevated temperature. (Xue, Z. et al. ACS Appl. Mater. Interfaces 10, 27141-27149 (2018).) Here, it was demonstrated that pure Na.sub.yCoO.sub.2 can be achieved from 03-Li.sub.xCoO.sub.2 when high structural Li preference regions are avoided. The phase equilibrium between Li.sub.0.94CoO.sub.2 and Na.sub.0.48CoO.sub.2 indicates that interconversion between Li and Na layered oxide is possible outside the range of phase equilibrium, which is a+b≥0.94 for Li and a+b≤0.48 for Na. Li.sub.0.40CoO.sub.2, Li.sub.0.50CoO.sub.2, and Li.sub.0.60CoO.sub.2 were prepared for the demonstration. The OCV curves went through a series of slopes and plateaus indicating the occurrence of ion exchange (FIG. 3A). After reaching equilibrium, Li.sub.0.40CoO.sub.2 and Li.sub.0.50CoO.sub.2 only showed Na diffraction peaks, whereas Li.sub.0.60CoO.sub.2 had both Li and Na diffraction peaks (FIG. 3B). Li contents a are 0.01 (˜98% Na purity), 0.05, and 0.24 confirmed by ICP-MS (FIG. 3C), which agree with the predicted compositions of Na.sub.0.4CoO.sub.2, Li.sub.0.04Na.sub.0.46CoO.sub.2, and Li.sub.0.25Na.sub.0.35CoO.sub.2, respectively, based on the phase equilibrium. The exchanged Na.sub.0.48CoO.sub.2 from Li.sub.0.48CoO.sub.2 was used for further characterization. STEM imaging showed a 5.5 Å interlayer distance confirming the Na.sub.0.48CoO.sub.2 phase (FIG. 3D). The uniform distribution of Na EDS signal on both the particle ensemble level (SEM, FIG. 3E) and the single particle level (STEM, FIG. 3F) was observed, indicating the completion of Na ion exchange with the structural Li.

[0044] Overcoming the Kinetic Barrier by Electrochemical Assisted Ion Exchange

[0045] Next, a strategy was demonstrated to avoid the inaccessible ion exchange pathway and realize the formation of Li.sub.0.94CoO.sub.2 from Na.sub.0.67CoO.sub.2 at a low Li ratio (1-1000 Li—Na) and small Li excess (18% excess of Li to target amount, Methods). The phase equilibrium predicted a pure Li phase at a+b≥0.94. However, in the range of 0.72<a+b<0.94, the final exchanged products did not follow the prediction due to the large kinetic barriers. Inspired by the established structure evolution pathway, the ion exchange was designed to start from the Na.sub.0.67CoO.sub.2 phase with enough vacancies and increase the cation content a+b to ˜0.94 by multiple electrochemical intercalations while maintaining 0.48≤y≤0.67 in the Na.sub.yCoO.sub.2 phase for fast ion exchange.

[0046] As shown in FIG. 4A, after the first phase equilibrium, the electrode was intercalated to the starting potential of Na.sub.0.67CoO.sub.2. Then the process was repeated until the electrode reached its full capacity. Due to the ongoing exchange, the potential went back to the equilibrium potential once the intercalation current was stopped. Moreover, the intercalation voltage profile also shows the same four plateaus, which correspond to the transformation from Na.sub.0.48CoO.sub.2 to Na.sub.0.67CoO.sub.2. After three times of intercalation, the total amount of alkali-metal ions reached 0.92 (a+b=0.92 for Li.sub.aNa.sub.bCoO.sub.2).

[0047] Even though the intercalation process is accompanied by a simultaneous ion exchange process, the chemical composition evolution during the whole process can be predicted using the Na component in the Na phase reflected by the electrochemical potential. The measured compositions at intermediate steps and the calculated compositions based on the evolution equation show excellent matching (FIGS. 4B-4C). This proves again that the ion exchange process is governed by the two-phase equilibrium in the whole range of 0.48≤a+b≤0.94. With the electrochemical assisted ion exchange, a complete exchanged lithium cobalt oxide (Li.sub.0.90Na.sub.0.02CoO.sub.2) was obtained. A minor 2% of Na, based on ICP-MS, was detected in the structure, which also existed for the ion exchange conducted with a high Li ratio and large excess (Li.sub.0.95Na.sub.0.016CoO.sub.2 was obtained from exchange in 5 M LiOH and LiCl solution (1:1) for 19 hours). The structural characterization shows pure XRD patterns of Li.sub.0.94CoO.sub.2 (FIG. 4D). The corresponding 4.7 Å interlayer spacing was also observed for Li.sub.0.90Na.sub.0.02CoO.sub.2 by HAADF-STEM which supports the successful conversion to Li.sub.0.94CoO.sub.2 (FIG. 4E).

[0048] Since the Na.sub.0.67CoO.sub.2 to Li.sub.0.94CoO.sub.2 conversion was done in 1-1000 Li—Na solution, it marks the excellent structural selectivity (4.5×10.sup.4, Methods) for layered oxide to enable Li extraction application with ˜98% Li purity. Moreover, it was demonstrated that the Li extraction can also be achieved using an even lower Li ratio (1-10000 Li—Na) (FIGS. 7A-7C).

[0049] Finally, all the accessible conversion pathways for Li and Na ion exchange in layered cobalt oxide were labeled (FIG. 5). All Li.sub.xCoO.sub.2 can be achieved from Li exchanging with Na.sub.yCoO.sub.2 given sufficient driving force and avoiding the kinetic-limited regions. However, only Na.sub.yCoO.sub.2 with y<0.48 can be achieved at large Na concentrations and excesses due to the extremely strong structural Li preference. These results indicate that the phase equilibrium between Li.sub.0.94CoO.sub.2 and Na.sub.0.48CoO.sub.2 plays a key role in determining the ion exchange pathway in all cation ranges.

[0050] This Example also showed that vacancy-dependent large thermodynamic Li preference can trigger the exchange of Na.sub.yCoO.sub.2 at an extremely low Li ratio (e.g. 1-10000) and small excess. This Example resolved general ion exchange pathways that Li.sub.0.94CoO.sub.2 nucleation initiated the ion exchange and were followed by surface reaction-limited and diffusion-limited exchange pathways at near equilibrium (e.g. 1-1000) and far from the equilibrium (e.g. 1 M Li) conditions, respectively. Guided by the understanding of the ion exchange mechanism, Na.sub.yCoO.sub.2 conversion from the parent Li.sub.xCoO.sub.2 was demonstrated for the first time, and Na.sub.yCoO.sub.2 conversion to Li.sub.0.94CoO.sub.2 was realized via electrochemical assisted ion exchange. This work opens new opportunities for ion exchange in predictive synthesis and Li extraction.

[0051] Methods

[0052] Sample Preparation

[0053] P3-Na.sub.0.67CoO.sub.2 and P2-Na.sub.0.67CoO.sub.2 were synthesized via a known solid-state method. (Lei, Y. et al., Chem. Mater. 26, 5288-5296 (2014).) Na.sub.2O.sub.2 (Alfa, 95%) and Co.sub.3O.sub.4 (Alfa, 99.7%) were mixed in a stoichiometric ratio of Na:Co=0.68:1 (a slight excess of Na) in an Ar glovebox. 30 min high-energy ball-milling was treated before pressing the mixture into a pellet. The transferring step from the ball-milling container to the press dies was finished in the Ar glovebox as well to minimize air contact. The pellet was heated at 535° C. for 16 h to obtain P3-Na.sub.0.67CoO.sub.2 and at 700° C. for 16 h to obtain P2-Na.sub.0.67CoO.sub.2. After cooling down to ˜300° C., the pellet was transferred into the Ar glovebox and stored for later use. Electrode slurries were prepared by mixing the active material, conductive carbon (Super P, MTI), and binder (polyvinylidene fluoride, MTI) in an 8:1:1 weight ratio, together with N-methyl-2-pyrrolidone (NMP, Sigma-Aldrich). The mass loading was 2-3 mg per electrode. Electrodes were made by coating slurries on the carbon cloth substrate.

[0054] Electrochemical Test

[0055] All the electrochemical tests (including the OCV tests) were conducted in the three-electrode system using SP-300 potentiostat (BioLogic) in the Ar glovebox. The potential for the commercial LiCoO.sub.2 in 1 M Li ACN charging is around 0.6 V versus the non-aqueous Ag.sup.+/Ag reference electrode (CH Instrument Inc.). Therefore, the potential of the reference electrode is around 3.0 V versus Ne/Na. All the plotted curves were manually shifted 3.0 V for better comparison. The counter electrodes LiFePO.sub.4, NaFePO.sub.4, or FePO.sub.4 were chosen depending on the major cation in electrolytes. Electrolytes were prepared by dissolving LiClO.sub.4 and (or) NaClO.sub.4 into 30 mL acetonitrile (CAN) according to different ratios. 500 mL 1-10000 Li—Na CAN was used for validating the electrochemical assisted ion exchange method. C/80 was used for collecting the galvanostatic curves of P3-Na.sub.yCoO.sub.2 and P2-Na.sub.yCoO.sub.2. C/40 and C/10 were used to prepare NayCoO.sub.2 electrodes and Li.sub.xCoO.sub.2 electrodes with different vacancies, respectively. C/10 was used in the electrochemical assisted ion exchange process.

[0056] Li.sub.0.94CoO.sub.2 and Na.sub.0.48CoO.sub.2 Phase Assignment

[0057] The newly appeared Li peak position matched well with Li.sub.0.94-1.0CoO.sub.2. (Ménétrier, M. et al., Journal of Materials Chemistry 9, 1135-1140 (1999).) Considering the low electronic conductivity of Li.sub.xCoO.sub.2 at 0.94≤x≤1, which will induce a high energy barrier for Li insertion during ion exchange in a low Li—Na ratio solution, the newly appeared Li phase was assigned to Li.sub.0.94CoO.sub.2 instead of Li.sub.1.0CoO.sub.2. (Ménétrier, M. et al., 1999; Dahéron, L. et al. Chem. Mater. 20, 583-590 (2008).) The new Na phase had an interlayer distance of 5.55 Å, which corresponds to Na.sub.yCoO.sub.2 at 0.3<y<0.5. (Viciu, L. et al. Phys. Rev. B 73, 174104 (2006).) The galvanostatic curve (FIG. 1A) gave y=0.48 based on the final voltage, which was also confirmed by inductively coupled plasma mass spectrometry (ICP-MS).

[0058] Equilibrium Equation and Evolution Equation

(1−x)f+(1−y)(1−f)=c,

where x is the Li component in the Li phase Li.sub.xCoO.sub.2, x=0.94 for the equilibrium condition and during evolution; y is the Na component in the Na phase Na.sub.yCoO.sub.2, y=0.48 for the equilibrium condition and 0.48<y<0.67 during evolution; f is the phase fraction of the Li phase; c is the total vacancy in the structure. The Li content a, and Na content b in the structure Li.sub.aNa.sub.bCoO.sub.2 (a, b are different than x, y in equilibrium Li and Na phases) after reaching equilibrium can be calculated by a=x×f and b=y×(1−f). During evolution, Na components y (y=0.64, 0.565, and 0.50 at plateau I, II, and IV, respectively) are determined via coulomb counting by comparing the plateau voltages of OCV curves and that of the galvanostatic curve (FIG. 1A).

[0059] Structural Selectivity

[0060] The structural selectivity is calculated based on the final composition Li.sub.aNa.sub.bCoO.sub.2 versus the Li—Na ratio in the system as (a/b)/ratio.

[0061] The Calculation of the Excess Amount of Li

[0062] The Li amount provided in 30 mL 1-1000 Li—Na ACN solution was 0.03 mmol. The exchanged Li amount in the 3 mg electrode with a final composition of Li.sub.0.90Na.sub.0.02CoO.sub.2 was 0.0254 mmol. The excess amount of Li was calculated as 18%.

[0063] X-Ray Diffraction

[0064] Synchrotron XRD measurements (0.1173 Å) were conducted at the 13-BM beamline of Advanced Photon Source. Intensities in FIG. 1C and FIG. 2B have been normalized to make the strongest diffraction peaks have the same intensity. X-ray diffractions of P2-Na.sub.2CoO.sub.2, converted Li.sub.0.90Na.sub.0.02CoO.sub.2, and converted Na.sub.yCoO.sub.2 were collected by using Rigaku MiniFlex 600 with a Cu Kα source. The Rietveld refinements were carried out using GSAS II. The instrument parameters were modified based on the “defaults for APS 30 KeV 11 BM”. Diffractions of single-phase Na.sub.0.57CoO.sub.2 and Na.sub.0.67CoO.sub.2 were first refined based on literatures. (Viciu, L. et al., 2006; Ono, Y. et al. Journal of Solid State Chemistry 166, 177-181 (2002).) The unit cell and atom coordination were refined. The obtained phase information as the reference was used for refining biphasic diffractions. Phase fraction was added for the biphasic diffraction refinement.

[0065] Scanning Transmission Electron Microscopy (STEM)

[0066] The top-view of equilibrium particles showing Li—Na phase separation and atomic-resolution images of the fully converted Li.sub.0.90Na.sub.0.02CoO.sub.2 were conducted by using the aberration-corrected scanning transmission electron microscope (STEM) JEOL ARM200CF at the University of Illinois at Chicago. The HAADF detector angle was 90-270 mrad to give Z contrast images. The low-angle annular dark-field detector angle ranged between 40 and 120 mrad. The energy dispersion for EELS (Gatan) was 0.15 eV/pixel with 0.1 s per pixel dwell time. EDS spectra imaging was acquired using an Oxford X-Max 100TLE windowless SDD detector. The cross-section views of the fully converted Na.sub.0.48CoO.sub.2 were conducted using the aberration-corrected scanning transmission electron microscope (STEM) JEOL ARM200CF at Northwestern University. EDS spectra imaging was acquired using a Dual SDD EDS detector.

[0067] Particles were removed from the electrodes after reaching the equilibrium by sonication and were drop-cast onto lacey carbon membrane-coated gold grids for top-view imaging. For the cross-section view imaging of the converted Li.sub.0.90Na.sub.0.02CoO.sub.2, after removing from the electrodes, particles were embedded into Poly/Bed 812 resin and cut into 90 nm thick slides using a ultramicrotome (Ultracut E, Reichert-Jung). For the cross-section view imaging of the converted Na.sub.0.48CoO.sub.2, micron-sized particles were removed from the electrodes, which were site-specifically cut and thinned using conventional focused ion beam scanning electron microscopy (FIB-SEM, FEI Helios NanoLab 600). Specifically, a ˜150 nm thick carbon layer and ˜1.2 μm thick platinum layer were initially deposited using a gas injection system (GIS) to protect the surface of the target particle. After removal of an approximately 8×2×4 μm section via in-situ lift-out using a W micromanipulator (Oxford Omniprobe 200), the lamella was thinned at initially 30 kV, 0.49 nA, and subsequently at 5 kV, 81 pA. Finally, the sample was cleaned at 2 kV and 28 pA to yield a ˜90 nm thick lamella.

[0068] Scanning Electron Microscopy (SEM)

[0069] Scanning electron micrographs were obtained on a Zeiss Merlin scanning electron microscope using a 20 kV accelerating voltage. EDS spectra imaging was acquired using an Oxford Ultim Max 100 EDS detector.

[0070] Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

[0071] 3% HNO.sub.3 (aq) was used as the diluting matrix, and all the measurements used either Thermo iCAP Q ICP-MS or Thermo iCAP RQ ICP-MS.

[0072] Before dissolving, each electrode was washed at least 6 times with 10 mL ACN solution each time to remove residual salts on the surface as completely as possible. 8 mL aqua regia was used to dissolve each electrode.

[0073] The word “illustrative” is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “illustrative” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Further, for the purposes of this disclosure and unless otherwise specified, “a” or “an” can mean only one or can be mean “one or more.” Embodiments of the inventions consistent with either construction are covered.

[0074] The foregoing description of illustrative embodiments of the invention has been presented for purposes of illustration and of description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and as practical applications of the invention to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.