Ultrathin Polyimide Coating for a Spinel LiNi_0.5Mn_1.5O₄Cathode and Its Superior Lithium Storage Properties under Elevated Temperature Conditions

In this study, we present the influence of polyimide (PI) coating concentration on the electrochemical properties of high voltage, spinel phase LiNi 0.5 Mn 1.5 O 4 cathodes, particularly under elevated temperature conditions. First, the adipic acid-mediated sol-gel technique was employed to synthesize sub-micron sized LiNi 0.5 Mn 1.5 O 4 particles, where Mn was in the 4+ state. Thermal polymerization was used to produce the PI coating from polyamic acid. The presence of the PI layer was confirmed by transmission electron microscopy and Fourier-transform infrared analyzes. All test cells delivered good cycleability under ambient temperature conditions, irrespective of the PI coating concentration, with a prominent plateau at 4.7 V vs. Li, whereas all test cells experienced the poorest electrochemical behavior under elevated temperature conditions except 0.3 wt.% PI. The 0.3 wt.% PI coated LiNi 0.5 Mn 1.5 O 4 phase delivered excellent cycleability with capacity retention of > 90% at 55°C. Poor compatibility and severe reactivity toward the electrolyte solution resulted in the poorest performance which was clearly evidenced by the scanning electron microscopy analysis and supported well by impedance studies after galvanostatic cycling.