Na-ion batteries (SIBs) require novel anode materials which have high ion storage capability and low working voltage to make SIBs competitive compared to Li-ion batteries (LIBs). From this perspective, we present the origin of improved electrochemical performances of phosphate doped hard carbon (P-HC) which can meet the two requirements mentioned above through combining experiment with the calculations of atomic/electronic structures. We disclosed that the capacity enhancement is accompanied by the changed voltage profile, which results from the introduced phosphate functional groups. The emerged redox peak turned out to be generated by the electrochemical reaction of Na ions near the phosphate-carbon environment. First-principles calculations elaborated that the phosphate introduction here generates electron holes near the Fermi level that is generally considered as evidence of p-type semiconductors to improve electronic conductivity. This expectation has been proven by comparatively measuring the electrical conductivities of pristine hard carbon and P-HC. A close investigation into charge distribution indicated that the electron hole is generated mainly by the higher reducibility of the doped phosphate than the surrounding carbon atoms in P-HC. This discovery well explains the underlying principles for the enhanced electrochemical performance of P-HC, thereby showing a way to design highly functional hard carbon structures toward higher capacity with Na ions.

The rapidly growing technological demand for lithium-ion batteries has prompted the development of novel cathode materials with high energy density, low cost, and improved safety. High voltage spinel, LiNi0.5Mn1.5O4 (LNMO), is one of the most promising candidates yet to be commercialized. The two primary obstacles for this material are the inferior electronic conductivity and fast capacity degradation in full cells due to the high operating voltage. By systematically addressing these limitations, we successfully develop a thick LNMO electrode with areal capacity loadings up to 3 mAh·cm−2. The optimized thick electrode is paired with a commercial graphite anode at both the coin cell and pouch cell level, achieving a full cell capacity retention as high as 72% and 78%, respectively, after 300 cycles. We attribute this superior cycling stability to careful optimizations of cell components and testing conditions, with a specific focus improving electronic conductivity and high voltage compatibility. These results suggest precise control of materials quality, electrode architecture and electrolyte optimization can soon support the development of a cobalt-free battery system based on a thick LNMO cathode (>4 mAh·cm2), which will eventually meet the needs of next-generation Li-ion batteries with reduced cost, improved safety, and assured sustainability.