Lepidocrocite-type Layered Titanate Structures: New Lithium and Sodium Ion Intercalation Anode Materials

The electrochemical characteristics of lepidocrocite-type titanates derived from K_0.8Ti_1.73Li_0.27O₄ are presented for the first time. By exchanging sodium ions for potassium, the practical specific capacity of the titanate in both sodium and lithium half cells is considerably enhanced. Although the gross structural features of the titanate framework are maintained during the ion exchange process, the symmetry changes because sodium occupies different sites from potassium. The smaller size of the sodium ion as compared to potassium and the change in site symmetry allow more alkali metal cations to be inserted reversibly into the structure during discharge in sodium and lithium cells than in the parent compound. Insertion of lithium cations takes place at an average of about 0.8 V vs Li⁺/Li while sodium intercalation occurs at 0.5 V vs Na⁺/Na, with sloping voltage profiles exhibited for both cell configurations, implying single-phase processes. Ex situ synchrotron X-ray diffraction measurements show that a lithiated lepidocrocite is formed during discharge in lithium cells, which undergoes further lithium insertion with almost no volume change. In sodium cells, insertion of sodium initially causes an overall expansion of about 12% in the <i>b</i> lattice parameter, but reversible uptake of solvent minimizes changes upon further cycling. In the case of the sodium cells, both the practical capacity and the cyclability are improved when a more compliant binder (polyacrylic acid) that can accommodate volume changes associated with insertion processes is used in place of the more common polyvinylidene fluoride. The ability to tune the electrochemical properties of lepidocrocite titanate structures by varying compositions and utilizing ion exchange processes make them especially versatile anode materials for both lithium and sodium ion battery configurations