Abstract

  With increasing demand of high energy density batteries for electric vehicles (EVs), Ni-rich NMC batteries become the trend. However, safety and costs of Ni-rich NMC cells have been a great concern in the EV industry. In contrast, Lithium manganese iron phosphate (LMFP) with olivine structure has attracted much attention due to its high voltage (~4V), long cycle life, low cost and high safety. In this article, I will introduce the novel NMC/LMFP composite technologies (i.e. blended and dual-layered electrodes) that can provide the best solution to high energy density and safety, low cost and long-life Li-batteries for EVs and energy storage system (ESS).

Keywords：Lithium-ion battery, Lithium Manganese Iron Phosphate(LMFP), Lithium nickel manganese cobalt dioxide (NMC), LMFP and NMC composite、dual-layered electrode

1. Introduction

     Since their introduction in 1990, lithium-ion batteries have made remarkable progress in performance. In the past 30 years, they have been widely used in products such as computers, communications and consumer electronics, and has had a great impact on human society. In recent years, in order to reduce human dependence on petrochemical and coal fuels, reduce pollution emissions, and alleviate global warming problems, various electric vehicles, solar and wind renewable energy have become the focus of development in the world. Due to the rich variety of positive and negative materials for lithium-ion batteries, lithium-ion batteries exhibit advantageous characteristics in different application scenarios. Today, the advanced small cylindrical "21700" battery has an energy density of 255 Wh/kg and 720 Wh/L and can support an electric vehicle with a range of 300 miles. And battery prices have dropped significantly from $1,000/kWh in 2006 to $120-140/kWh in 2021. lithium-ion batteries are already the technology of choice for battery electric vehicles (BEV) and energy storage systems (ESS). Nevertheless, researchers continue to improve lithium-ion batteries from three perspectives: (1) increasing energy density and lifetime, (2) reducing costs, and (3) increasing charging rates; these goals must be achieved without sacrificing battery manufacturability and safety. At present, Lithium nickel manganese cobalt dioxide (NMC) materials and batteries have high electric capacity, but their cycle life and safety are not good. Recently, NMC lithium battery fire accidents have occurred frequently (Figure 1), and various car manufacturers have also put forward higher requirements for safety.

     According to the 2021 United States Geological Survey (USGS), the world's known lithium reserves are 86 million tons, mainly distributed in South America (57%), the United States (9%) and Australia (7%). Known lithium mines are enough to produce up to 792 trillion (7.92E14) watt-hours (792,000GWh) of lithium-ion batteries. The world currently produces approximately 90 million new cars a year. Assuming that all of them are replaced by 100KWh electric vehicles, 9000GWh lithium-ion batteries are needed every year. If the lithium is not recycled, it can last for 88 years. In the real world, the recycling of battery-grade materials will be implemented gradually, so in theory, lithium shortages will never happen. Although there are abundant lithium resources to support the development of global lithium batteries, cobalt in NMC cathode materials will be a problem. According to the US Geological Survey in 2021, there are 7.1 million tons of cobalt reserves in the world, mainly in the Congo (50%) and Australia (20%). Last year, the world produced 140,000 tons of cobalt, while the demand was 170,000 tons of cobalt. 24% of them are used in lithium batteries. Lithium and cobalt resources are unevenly distributed in the world, which intensifies global competition for resources. In the past year, the surge in lithium, cobalt and nickel has caused the cost of NMC to soar, which is not conducive to the development of the electric vehicle industry. Based on the price of lithium, cobalt, and nickel in 2021, the impact of a 10% increase in lithium, cobalt, and nickel on the cost of battery packs with different cathode materials is estimated and shown inFigure 2. It can be seen from the figure that the NMC has the highest increase (~ 2.9%) while LiFePO4 (LFP) was the lowest (~0.7%).

    In this article, I will introduce not only LMFP cells but NMC-LMFP composite cells with blended and double-layer electrode structure, which can provide the best battery solution for electric vehicles and energy storage systems with high energy density, safety, low cost and long life.

2. Lithium manganese iron phosphate (LMFP) cells

     Lithium manganese iron phosphate (LMFP) with an olivine structure has attracted much attention due to its high voltage (~4 V), long cycle life, and high safety. In addition, due to the extremely rich reserves of iron and manganese in the earth's crust, the cost of LMFP is low. As shown in Figure 3, the LMFP exhibits a high discharge capacity of 150 mAh/g at 0.1C and excellent rate performance (i.e., 145 mAh/g for 1C, 131 mAh/g for 5C, and 118 mAh/g for 10C). The first discharge plateau at 4V is related to the reduction of Mn3+ in LMFP, while the second discharge plateau at 3.5V the reduction of Fe3+. The discharge capacity of LMFP at 0.5C at low temperature (-20°C) is about 100 mAh/g, which is about 71% of the capacity at 25°C, as shown in Figure 3.

   Due to the higher voltage, LMFP batteries have ~20% higher energy density than lithium iron phosphate (LFP) batteries. ITRI has developed an energy-typed LMFP battery (~3.5Ah) with an energy density of 185 Wh/kg (electrode density 2.2 ~2.3g/c.c.; electrode surface capacity ~4.5 mAh/cm2), 0.5C-1C cycle life over 3000 times (capacity retention ~78%) (Fig. 4); and power-typed LMFP batteries with discharge and charge powers as high as 12C and 3C, respectively, and cycle life over 4000 cycles (Fig. 5).

    The high safety of LMFP is manifested in its high thermal stability with the electrolyte. Specifically, when comparing the reactions of delithiated LMFPs and NMCs with electrolytes, LMFPs performed better in terms of the highest reaction temperature and least heat release, as shown in Figure 6. Compared with NMCs, NCM-LMFP mixtures have higher decomposition temperatures with 40% to 60% lower exotherm (Fig. 6).

    In this context, nickel-rich NMC-LMFP composites can achieve lithium-ion batteries with balanced energy density, power, cycle life, safety, and cost for electric vehicles, eVTOL, and energy storage systems.

3. LMFP-rich NMC-LMFP composite battery

     ITRI developed NMC-LMFP composite batteries rich in LMFP (>50 wt.%). Figure 7 is the charge-discharge curve of the 80LMFP+20NMC blend battery, and the energy density of the 3.5Ah battery is 200 Wh/kg. The battery retained 70% capacity after 1893 0.5C charge-1C discharge cycles at room temperature and 83% after 781 cycles at 45C. The battery passed the nail penetration test and the maximum battery temperature is 80C. As we further increased the NMC ratio to 40% (Fig. 8), the energy density of the 3.5Ah 60LMFP+40NMC blend battery increased to 215 Wh/kg. After 1323 0.5C charge-1C discharge cycle tests at room temperature, the battery retained 87% of its capacity. The battery also passed the nail penetration safety test with a maximum battery temperature of 90C.

    Furthermore, we found that NMC/LMFP bilayer batteries (US Patent 11228028) have better power and cycle life than blend batteries. Specifically, the bilayer battery with the upper layer of NMC has the best discharge power and cycle life at room temperature and -20 °C, as shown in Figure 9. The basic mechanism can be explained by the differential capacity dQ/dV curve shown in Figure 10. During the charging process, the oxidation process of the 60LMFP//40NMC double-layer battery with LMFP on the upper layer and the 60LMFP+40NMC blend battery was carried out in three stages: Fe → NMC→ Mn, while the 40NMC//60LMFP double-layer battery with the upper layer of NMC underwent two stages of (Fe & NMC) → Mn. During the cycling test, the capacity loss of the 60LMFP//40NMC battery and the 60LMFP+40NMC blend battery is due to the decay of NMC. In contrast, the NMC redox peaks of the 40NMC//60LMFP cells decayed rather slightly with cycling. The better cyclability of the 40NMC//60LMFP double-layer battery was due to its higher oxidation reaction potential (i.e., overvoltage of Fe2+ and Mn2+ oxidation peak), so that LMFP (Fe2+) and NMC were oxidized at the same time, so during the charging process the stress exerted on the NMC was smaller, and the NMC life was prolonged. It is particularly emphasized here that all LMFP-rich LMFP-NMC composite batteries passed the nail penetration safety test.

4. NMC-rich NMC-LMFP composite battery

    ITRI has also developed NMC-LMFP blend and bilayer electrode structure batteries rich in NMC (>50 wt.%). Figure 11 shows that the 80NMC+20LMFP blend battery with an energy density of 200 Wh/kg passed the nail penetration safety test, while the corresponding NMC battery failed. Importantly, we found that the cycle life of the 80NMC811+20LMFP blend battery was improved compared to the corresponding NMC811 battery (Figure 12). The better cyclability of the 80NMC+20LMFP blend battery can be explained by the fact that the NMC811 in the blend battery underwent less phase transition (i.e. H2→H3) at high voltage (~4.1V) than in the NMC battery, as shown in the differential capacity dQ/dV curves of Figure 12.

   Most strikingly, we found that the bilayer NMC-LMFP battery has better power (Figure 13) and safety (Figure 14) than the blend battery. Figure 14 is a stress safety test where the needle penetration speed reduced to 1 mm/sec. It shows that the voltage of the blend battery droped to 0V instantaneously and the maximum temperature reached 350C during the nail penetration process, while the voltage of the double-layer battery was maintained at 3.25 to 2V for 50 seconds and the maximum temperature reached only 90C.

    As the cell energy density and size increased, we observed that the bilayer cells with upper LMFP (i.e. 20LMFP//80NMC) seemed to have better safety than (80NMC//20LMFP). As shown in Figure 15, the 20LMFP//80NMC811 battery did not catch fire and only released thick smoke during the nail penetration test, while the 80NMC811//20LMFP and blend batteries ignited instantaneously.

   Furthermore, the bilayer 80NMC811//20LMFP cell with an energy density of 220 Wh/kg exhibited very good power capability at rates up to 5C, as shown in Figure 16.

 5.Conclusion

     LFP materials and batteries have low cost, long life and high safety, but their voltage and energy density are low; while NMC batteries have high energy density, but relatively poor life and safety, and high cost, the above two batteries each has its pros and cons. LMFP can not only replace LFP to increase battery density, but also can be composited with NMC to provide the best battery solution with high energy density, high safety, low cost and long life.