A New Sodium Calcium Cyclotetravanadate Framework: "Zero-Strain" during Large-Capacity Lithium Intercalation

"Zero-strain" materials with little lattice strain and volume change during long-term cycling are ideal electrode choices for long-life lithium-ion batteries. However, the very limited "zero-strain" materials explored generally show small capacities (<200 mAh g(-1)), and the origin of "zero-strain" is still unclear. Here, Na2Ca(VO3)(4) (NCVO) nanowires are explored as a new anode material capable of keeping single-phase-transition "zero-strain" during large-capacity (381 mAh g(-1)) Li+ intercalation. NCVO owns a crystal structure with isolated [V4O12](4-) tetracycles separated by large-sized NaO6 octahedra and CaO8 square antiprism decahedra, generating large-sized quadrilateral and hexagonal channels (approximate to 3.6 angstrom). During lithiation, two-electron transfer per vanadium is accomplished, introducing a large amount of Li+ into interstitial sites and increasing the size of reduced vanadium ions. The former and latter expansion effects are eliminated by the superior volume-buffering capabilities of the sufficiently large interstitial sites and electrochemical inactive Na-/Ca-based polyhedra, respectively, thus achieving "zero-strain" with the maximum volume variation of only 0.039% and mean strain of only 0.060%. Therefore, the NCVO nanowires exhibit exceptional cyclic stability, as demonstrated by 93.8%/93.2%/94.7% capacity retention over 2000/2000/7000 cycles at 1C/2C/10C. The understanding of the crystal-structural features for "zero-strain" provides a guide for the future designs of "zero-strain" energy-storage materials.