Ion-Exchanged Phosphomolybdic Acid Interfacial Modification Enhances the Electrochemical Performance of LiNi0.9Mn0.1O2

With the Keggin structure PMo12O403– of phosphomolybdic acid (PMA) taken into account, the interfacial physicochemical property of PMA was adjusted through Li-ion exchange, and then Li-exchanged PMA was adopted to modify the cathode material LiNi0.9Mn0.1O2 (NM91) via the solid-phase method, aiming at improving the rate performance and cycling stability of the cathode. The optimal ion-exchanged PMA (PMA-e2)-modified NM91 (named 91PMA-e2) shows an initial discharge capacity of 216.6 mAh g–1 at 0.1 C and a capacity retention of 84.8% after 100 cycles (1 C and 2.8–4.5 V) as well as the best rate performance, which is superior to those of pristine NM91. With characterization by X-ray diffraction (XRD), X-ray photoelectron spectroscopy, high-resolution transmission electron microscope, in situ XRD measurements, etc., it indicates that PMA-e2 modification can reduce the Li/Ni mixing degree but enlarge the thickness of the lithium layer (TLiO6) and facilitate Li+ diffusion. The results suggest that the Keggin structure of PMA as well as the acidity property and priority adsorption toward water molecules are associated with the Li-ion exchange degree. In combination with the depth-profiling XPS measurement of the spent samples experienced in 300 cycles, it illustrates that spent 91PMA-e2 possesses higher amounts of LiF and Ni3+ but a lower amount of LixPOyFz, which are due to the coating layer generated from the Keggin structure of PMA-e2. Such a Keggin structure has the capacity to trap trace amounts of water and inhibit the hydrolysis reaction of PF5, consequently maintaining the stable structure of the cathode and achieving superior cycling retention.