Use of a pyrosulfate-boron trifluoride composite metal salt in an electrolyte solution. The use of the pyrosulfate-boron trifluoride composite metal salt having at least one structure is added to an electrolyte solution at an addition amount of 0.1 wt % to 15.0 wt %. The pyrosulfate-boron trifluoride composite metal salt is obtained by means of the reaction of a pyrosulfate and boron trifluoride gas or a boron trifluoride complex. A pyrosulfate-boron trifluoride composite lithium salt is further applied to a lithium-ion secondary battery including a negative electrode containing an active material with a specific surface area of 0.1 m.sup.2/g to 20 m.sup.2/g.

The present invention relates to compounds and their nonlinear optical (NLO) crystals of A.sub.3B.sub.11P.sub.2O.sub.23 (A=K, Rb, Cs, NH.sub.4), their producing method and uses thereof. The series of compounds have a chemical formula of A.sub.3B.sub.11P.sub.2O.sub.23 (A=K, Rb, Cs, NH.sub.4), which are namely K.sub.3B.sub.11P.sub.2O.sub.23, Rb.sub.3B.sub.11P.sub.2O.sub.23, Cs.sub.3B.sub.11P.sub.2O.sub.23 and (NH.sub.4).sub.3B.sub.11P.sub.2O.sub.23. The series of NLO crystals having the chemical formula of A.sub.3B.sub.11P.sub.2O.sub.23 (A=K, Rb, Cs, NH.sub.4), belong to rhombohedral crystal system, and have a space group of R3, crystal cell parameters of a=b=10.016(5)-12.591(5) , c=12.105(6)-14.905(6) , Z=3. A.sub.3B.sub.11P.sub.2O.sub.23 (A=K, Rb, Cs, NH.sub.4) compounds were prepared by a solid-state reaction method or a hydrothermal method, and A.sub.3B.sub.11P.sub.2O.sub.23 (A=K, Rb, Cs, NH.sub.4) NLO crystals were prepared by a high-temperature solid-state reaction method, a hydrothermal method, or a solution method. T They meet the requirements for the frequency conversion of UV wavelength lasers and could be used to prepare nonlinear optical devices.