Types of NMC (111, 442, 532, 622, 811)
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In the automotive battery industry, one of the most successful Li-ion chemistries is the cathode combination of nickel manganese cobalt oxide (NMC). LiNi1-x-yMnxCoyO2 (NMC) has similar or higher specific capacity than LCO and similar operating voltage while having lower cost since the Co content is reduced. LiNi1/3Mn1/3Co1/3O2 (NMC-111) is the most common form of NMC and is widely used in the battery market. Another successful combination of NMC is LiNi0.5Mn0.3Co0.2O2 (NMC-532). Other combinations using various amounts of metals are possible.
For NMC, nickel is known for its high energy density but poor stability. Manganese has the benefit of forming a spinel structure to achieve low internal resistance, but gives a low specific energy. Combing the two metal elements can improve each other’s merits. NMC-based battery technology is also well-suited for EV applications due to having the lowest self-heating rate. There is a move towards NMC-blended Li-ion chemistry as the system can be built economically and it achieves good overall performance. The three active components of nickel, manganese and cobalt can easily be blended to suit a wide range of applications for automotive and energy storage systems (EES) that need frequent cycling. NMC-111, NMC-442 and NMC-532 are currently the-state-of-the-art cathode materials for LIBs. In the near future, Ni-rich NMC cathode materials (NMC-811, NMC-622) under development will likely be adopted in the automotive industry owing to their higher specific energy and lower cost. However, although Ni-rich NMC can efficiently enhance the specific energy, it is very hard to exceed its theoretical limitation (350 Wh kg-1 at a cell level).
In the long term, high-voltage spinel LiNi0.5Mn1.5O4 (HV-spinel) could be a promising alternative as a next-generation high-energy cathode material for EVs. HV-spinel possesses a high operating voltage at 4.7 V and a specific capacity of 130 mAh g-1, which leads to a specific energy of around 580 Wh kg-1 that can be obtained at the cathode-level. Although it shows a modest energy improvement, HV-spinel also has a large attraction due to facile synthesis, low cost, environmental friendliness, good safety and excellent rate capability owing to both high electron and ionic (Li+) conductivity. In particular, the rate capability of disordered HV-spinel phase (space group Fd-3m) is several orders of magnitude higher than that of the ordered one (space group P4332). However, such material suffers from some drawbacks such as severe capacity fading at elevated temperature (60 oC) and the electrolyte decomposition owing to higher operating voltage. Therefore, a high-voltage electrolyte needs to be developed for developing HV-spinel applications in the future.
High-energy NMC (HE-NMC) layered-layered composite materials, with the general formula xLi2MnO3.(1-x)LiMO2 (M=Ni, Mn, Co), are another alternative cathode which may become practical in the longer term (Figure 5-6). During the past several years, HE-NMC materials have attracted a large amount of attention and have raised considerable interest for the automotive industry because they exhibit the highest specific energy (~900 Wh kg-1) among all the cathode materials. HE-NMC materials are composed of LiMO2 (M=Ni, Mn, Co) and Li2MnO3. In such a composite structure, the layered LiMO2 component can be stabilized by the structurally compatible Li2MnO3 component. This allows a much larger degree of delithiation than that normally possible with pure layer LCO (Li1-xCoO2, x(max)=0.5).
In the cutoff voltage range of 2.0-4.4 V vs. Li+/Li, LiMO2 is the only electrochemically active component, since Li2MnO3 is inactive as the manganese ions are already tetravalent and cannot be further oxidized. In this regard, the main function of Li2MnO3 is to stabilize the LiMO2 layered structure by providing Li+ ions to the active LiMO2 component. However, when the voltage is increased to 4.4-4.6 V, Li2MnO3 becomes active and capacities above 250 mAh g-1 can be theoretically obtained. In the higher voltage range, the electrochemically active MnO2 phase will be generated owing to the removal of Li2O from Li2MnO3. Despite the favorably high capacity, HE-NMC still suffers from poor cycling stability, seriously limiting its practical application in the EV industry. This is mainly attributed to the extensive removal of Li2O from Li2MnO3, resulting in damage to the electrode surface and increased impedance, especially at high current densities. The severe voltage fading is probably ascribed to the transition towards a spinel phase when cycling at a cutoff window of 2.0 and 4.6 V. Besides cycling performance, low electronic conductivities and low tap densities need to be enhanced before HE-NMC could be considered as a potential battery technology for next-generation EV applications.
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