Transition metal-doped Ni-rich layered cathode materials for durable Li-ion batteries

Abstract

Doping is a well-known strategy to enhance the electrochemical energy storage performance of layered cathode materials. Many studies on various dopants have been reported; however, a general relationship between the dopants and their effect on the stability of the positive electrode upon prolonged cell cycling has yet to be established. Here, we explore the impact of the oxidation states of various dopants (i.e., Mg²⁺, Al³⁺, Ti⁴⁺, Ta⁵⁺, and Mo⁶⁺) on the electrochemical, morphological, and structural properties of a Ni-rich cathode material (i.e., Li[Ni_0.91Co_0.09]O₂). Galvanostatic cycling measurements in pouch-type Li-ion full cells show that cathodes featuring dopants with high oxidation states significantly outperform their undoped counterparts and the dopants with low oxidation states. In particular, Li-ion pouch cells with Ta⁵⁺- and Mo⁶⁺-doped Li[Ni_0.91Co_0.09]O₂ cathodes retain about 81.5% of their initial specific capacity after 3000 cycles at 200 mA g^-1. Furthermore, physicochemical measurements and analyses suggest substantial differences in the grain geometries and crystal lattice structures of the various cathode materials, which contribute to their widely different battery performances and correlate with the oxidation states of their dopants.

Fig. 1. Cycling of NC90, Mg-NC90, Al-NC90, Ti-NC90, Ta-NC90, and Mo-NC90 cathodes in half cells within the voltage range of 2.7–4.3 V versus Li⁰/Li⁺ and full cells within the voltage range of 3.0 - 4.2 V versus graphite.

a The first charge–discharge cycle curves at 0.1 C (18 mA g⁻¹) and 30 °C. Cycling at 0.5 C over 100 cycles b at 30 °C, and c at 60 °C. d Cycling of NC90, Mg-NC90, Al-NC90, Ti-NC90, Ta-NC90, and Mo-NC90 cathodes in pouch-type full cells within the voltage range of 3.0–4.2 V versus graphite at 1 C (200 mA g⁻¹) and 25 °C. e Cross-sectional images of recovered NC90, Mg-NC90, Al-NC90, Ti-NC90, Ta-NC90, and Mo-NC90 cathodes in the discharged state of 2.7 V after 1000 cycles at 100% DOD.

Fig. 2. Correlation between primary particle morphology and full cell performance.

a Cross-sectional SEM images of as-synthesized cathode particles that show the morphology and orientation differences between the polygonal grains of NC90, Mg-NC90, and Al-NC90, and the elongated grains of Ti-NC90, Ta-NC90, and Mo-NC90. Quantitative analysis results of the cathode grains and their orientations. b Relationship between aspect ratio and relative grain orientation, c aspect ratio (length/width), and d grain size as a function of oxidation state. Summaries of the capacity retention values after 1000 cycles as functions of average e angle of primary particle, f aspect ratio, and g grain size.

Fig. 3. HPPC testing and mechanical stability.

a DCIR (direct current internal resistance) graphs of the six cathodes as a function of SOC at 1st and 50th cycle. b Cross-sectional SEM images of NC90, Mg-NC90, Al-NC90, Ti-NC90, Ta-NC90, and Mo-NC90, charged to 4.5 V after 50 cycles.

Fig. 4. Summaries of crystal structures.

a a-axis lattice parameters, b c-axis lattice parameters, and c cation mixing through the interchange of Li⁺ and Ni²⁺ between layers. d The ratio of Ni²⁺/Ni³⁺ at the cathode surfaces as determined by XPS.

Fig. 5. The high oxidation states of dopants promote the Li/TM ordered structure.

a, b HAADF TEM images and derived structural models of Al-NC90 and Ta-NC90, respectively. c–g Bright-field images and corresponding SAED patterns of single primary particles of Mg-NC90, Al-NC90, Ti-NC90, Ta-NC90, and Mo-NC90. h Calculated [100] zone axis diffraction patterns for the normal layered structure and ordered superlattice structure. i Post-mortem analysis of a discharged Ta-NC90 cathode after 3000 cycles using TEM. Low magnification TEM image of a cycled Ta-NC90 particle and corresponding electron diffraction pattern (inset). j Schematic comparing the relative structural stability of the normal layered and ordered structures in highly delithiated states and after long-term cycling.

Fig. 6. Optimized electrochemical performance via control of the cobalt to manganese ratio of the Ni-rich layered cathode.

a Comparison of the electrochemical performances of cathodes featuring randomly oriented primary particles and high Co content (NCA80, NCM811, NCA90, NCM90, and NCMA90); radially aligned primary particles, an ordered structure, and high Co content (Ta-NC90); and radially aligned primary particles, an ordered structure, and low Co content (4%) (Ta-NCM90); in pouch-type full cells. Cycling performances of Ta-NC90, Ta-NCM900703, Ta-NCM900505, and Ta-NM9010 cathodes in half cells within the voltage range of 2.7–4.3 V versus Li⁰/Li⁺: b first charge–discharge cycle curves at 0.1 C (18 mA g⁻¹) and 30 °C; c cycling over 100 cycles at 0.5 C (90 mA g⁻¹) and 30 °C and d normalised capacity.