Na(Co,Ti)O2 Cathodes for Na-Ion Batteries: A Proper Choice

Abstract

Increased attention to sodium-containing materials during the last several years is caused by the rapid development of sodium-ion batteries (NIBs), considered as a potential successor for lithium-ion batteries (LIBs) [1]. Especially layered sodium oxides have gained large interest due to their potential applicability as electrode materials (both, as cathodes and anodes). Similar to LiCoO2 in LIBs, NaCoO2 shows an immense potential as a cathode in NIBs. A partial replacement of Co by Ti in a Na(Co,Ti)O2 cathode can stabilize the crystal structure during (de)sodiation and reduce the number of phase transformations. Although the phase diagram of the Na-Co-Ti-O system has not been studied yet, existence of single-phase and multi-phase regions in dependence on the Na-content and temperature is reported in the literature [2]. Typically, layered Na,Co,Ti-oxides adopt three very close structure types P2, O3 and P3, which differ in the oxygen surrounding for Na-cations (octahedral or prismatic) and in the number of the formula units in the crystal unit cell (2 or 3). For application in Na-batteries, however, a trial-and-error approach is mostly needed to evaluate a composition with outstanding electrochemical performance.The redox activity of Co and Ti cations is usually characterized by two well-separated cell potentials. The Co-oxidation (Co2+ → Co3+ and Co3+→ Co4+) occurs mostly at potentials between 3 and 4 V vs. Na+/Na, qualifying the material as cathode while Ti4+/Ti3+ redox process takes place below 1.5 V, as common for anode materials. Based on our experimental studies of layered Na,Co,Ti-oxides with different compositions as NaCo0.5Ti0.5O2 (O3) [3], Na0.65Co0.5Ti0.5O2 (P3) [3], Na0.67Co0.33Ti0.67O2 (P2- and O3-structures) [4] and Na0.8Co0.8Ti0.2O2 (P2) [5] as well as on literature report [6], we can conclude that the most stable cycling behavior can be realized, if the composition in the initial (discharged) state contains mainly Co3+ and not Co2+. The reason for this is a first-order spin-state transition of high-spin HS-Co2+ to low-spin LS-Co3+ and back upon (de)sodiation, which is accompanied by a phase transformation, leading to a huge potential hysteresis of about 2 V. This hysteresis only slightly depends on the temperature between 0 °C and 40 °C. Among the compositions mentioned above, Na0.8Co0.8Ti0.2O2 with the P2-structure is the best candidate for application as a cathode in Na-batteries. However, a deep discharge of the material below 1.6 V to reach the Na1.0Co0.8Ti0.2O2 composition is detrimental as well, since some amount of Co2+ appears, negatively influencing the cycling behavior.