Li[Ni0.9Co0.1]O2 (NC90), Li[Ni0.9Co0.05Mn0.05]O2 (NCM90), and Li[Ni0.9Mn0.1]O2 (NM90) cathodes are synthesized for the development of a Co‐free high‐energy‐density cathode. NM90 maintains better cycling stability than the two Co‐containing cathodes, particularly under harsh cycling conditions (a discharge capacity of 236 mAh g−1 with a capacity retention of 88% when cycled at 4.4 V under 30 °C and 93% retention when cycled at 4.3 V under 60 °C after 100 cycles). The reason for the enhanced stability is mainly the ability of NM90 to absorb the strain associated with the abrupt anisotropic lattice contraction/extraction and to suppress the formation of microcracks, in addition to enhanced chemical stability from the increased presence of stable Mn4+. Although the absence of Co deteriorates the rate capability, this can be overcome as the rate capability of the NM90 approaches that of the NCM90 when cycled at 60 °C. The long‐term cycling stability of NM90 is confirmed in a full cell, demonstrating that it is one of the most promising Co‐free cathodes for high‐energy‐density applications. This study not only provides insight into redefining the role of Mn in a Ni‐rich cathode, it also represents a clear breakthrough in achieving a commercially viable Co‐free Ni‐rich layered cathode.
density and long service life. General electromobility, which is considered necessary for the reduction of carbon emissions, requires the current EV market to grow exponentially; however, EV technology is confronted with significant performance and economic challenges, such as, limited driving range and battery durability and high battery costs, respectively. [1][2][3] These issues are directly related to the limitations of the LIBs powering EVs. Hence, improvements in the energy density and cycling stability of LIBs, as well as cost reduction, are prerequisites for envisioned general electromobility. Efforts have focused on developing high-capacity cathode materials because the overall performance of current LIBs is dictated by their cathodes. The capacity and cycle life of cathodes are generally inferior to those of graphite anodes (e.g., Li[Ni 0.8 Co 0.15 Al 0.05 ] O 2 and graphite have capacities of 200 and 360 mAh g -1 , respectively). Among the viable cathode materials for LIBs, layered Ni-rich lithium-nickel-cobalt-aluminum oxides, Li[Ni x Co y Al z ]O 2 (NCA), are the most promising materials for EV LIBs owing to their high theoretical capacity (278 mAh g -1 ) and good rate performance. [4,5] Recently, Tesla Motors adopted NCA materials in LIBs to power their Models S, X, and 3, which are capable of operating for 400-550 km per single full charge. However, to compete against internal combustion engine vehicles, EVs should have a driving range exceeding 600 km per single charge, which can be achieved by increasing the energy density of the cathode. Increasing the energy density of an NCA cathode requires increasing its relative fraction of Ni. [6] However, a highly delithiated NCA cathode, when charged to 4.3 V, suffers from structural degradation at the cathode particle surfaces owing to reaction between unstable Ni 4+ and the electrolyte that forms a deleterious NiOlike rock salt impurity phase. [7][8][9] Furthermore, Ni-rich layered cathodes (relative Ni fraction ≥ 0.8) undergo abrupt lattice contractions in the c-axis direction caused by H2→H3 phase transitions (at ≈4.15 V), which generate microcracks. [9][10][11][12][13][14][15] The severity of the microcracking increases with increasing Ni content. [14] The microcracks cause the electrolyte to penetrate the particle interior and attack inner primary particles, causing structural degradation, which leads to capacity fading and eventually to catastrophic mechanical failure. [6,7,[12][13][14][15] Moreover, this degradation triggers oxygen release from the host structure, giving riseThe Nb doping of Li[Ni 0.855 Co 0.13 Al 0.015 ]O 2 (NCA85) modifies its primary particle morphology to allow precise tailoring of its microstructure. The Nb dopant (1 mol%) elongates the primary particles and aligns them in the radial direction, creating a configuration that effectively dissipates the abrupt internal strain caused by H2↔H3 phase transitions near the charge end. The negation of the internal strain substantially improves the long-term cycling stability achieved...
transportation emissions account for nearly one-quarter of all greenhouse gases. [1] However, the current fleet of EVs, mainly powered by lithium-ion batteries (LIBs), still falls short of performance standards, especially in driving range per charge, that are required for broad consumer appeal. A driving range comparable to that of an ICEV requires a substantial increase in the energy density of LIBs, whose capacity is largely limited by the cathode. [2][3][4] Archetypal cathodes for LIBs deployed in current EVs are layered Li[Ni x Co y (Al or Mn) 1−x−y ]O 2 (Al = NCA or Mn = NCM) oxide materials. [4][5][6][7] Both cathodes were derived from LiNiO 2 , which has a high theoretical capacity of 270 mAh g −1 . Multiple phase transitions during delithiation quickly deteriorate the reversible capacity of LiNiO 2 ; this inherent structural instability renders the cathode unsuitable for EV applications. NCA cathodes were developed by introducing Co 3+ and Al 3+ to LiNiO 2 to prevent multistep phase transitions and stabilize the structure, resulting in the Li[Ni 0.8 Co 0.15 Al 0.05 ]O 2 cathode, which currently powers the Tesla Model S. [8] In another example, Ni was partially replaced with Co and Mn to develop Li[Ni 1/3 Co 1/3 Mn 1/3 ]O 2 ; this cathode exhibited excellent capacity retention and thermal stability but its capacity was limited to 160 mAh g −1 . [9] To compensate for inferior capacity, Ni content was increased to x = 0.6; this cathode is also widely commercialized. Although both NCA and NCM cathodes are adequate, the energy density of 350 Wh kg −1 required to provide a drive range threshold of 300 miles per charge requires a new class of layered oxide cathodes. As Ni content exceeds x = 0.8, NCA and NCM cathodes are plagued by increasingly compromised battery life and thermal safety, due to rapid capacity fading and an abundance of unstable Ni 4+ species, as observed in LiNiO 2 . [4,[10][11][12][13] To overcome the inherent instability of Ni-rich NCM and NCA cathodes, we propose a new type of layered oxide cathode, Li[Ni x Co y W 1−x−y ]O 2 . Previous work indicates that W doping of LiNiO 2 substantially improved its cycling stability without sacrificing energy density. [14,15] In this study, we show that replacement of Al ions with W ions in a Ni-rich NCA layered oxide cathode markedly modifies the cathode microstructure through particle refinement and greatly improves the cycling stability of the cathode. We compare the electrochemical performance of the Li[Ni 0.9 Co 0.09 W 0.01 ]O 2 cathode (NCW90) to the well-characterized Li[Ni 0.885 Co 0.1 Al 0.015 ]O 2 (NCA89) to demonstrate its superior structural and thermal stability compared to the commercialized NCA cathode.Substituting W for Al in the Ni-rich cathode Li[Ni 0.885 Co 0.10 Al 0.015 ]O 2 (NCA89) produces Li[Ni 0.9 Co 0.09 W 0.01 ]O 2 (NCW90) with markedly reduced primary particle size. Particle size refinement considerably improves the cathode's cycling stability such that the NCW90 cathode retains 92% of its initial capacity after 1000 cycles (c...
In article number 1902698, Chong S. Yoon, Yang‐Kook Sun and co‐workers develop a new type of Ni‐rich layered cathode, Li[Ni0.9Co0.09W0.01]O2 for next‐generation electric vehicles. Substituting W for Al in the NCA cathode reduces primary particle size. The particle size refinement improves the cycling stability of the cathode by suppressing microcrack propagation and preventing particle fractures. Thus, the cathode delivers a high energy density with a long battery life.
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