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https://www.linkedin.com/pulse/welion-360whkg-solid-state-battery...

  1. ccdavid
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    09/02/24
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    Post #: 72316858


    https://www.linkedin.com/pulse/welion-360whkg-solid-state-battery-analysis-wan-sodium-ion-battery

    Welion 360Wh/kg Solid State Battery Analysis

    Jerry Wan-Sodium Ion Battery

    Jerry Wan-Sodium Ion Battery

    Battery Expert at Biwatt

    Published Jul 24, 2023

    First of all, from Welion's user manual to analyse the basic information of this 360Wh/kg battery cell, its rated capacity is 110Ah, and the rated voltage is 3.31V (note that this is the key point), which is much lower than the general NMC lithium battery cell's 3.6~3.75V. This is due to the use of silicon carbon anode contains more silicon, because the discharge voltage platform of silicon is higher, at about 0.4V, 0.3V higher than graphite's 0.1V, and accordingly, the rated voltage of its battery cell will drop by about 0.3V (the average voltage of the general 90% nickel NMC with a graphite anode is about 3.62V on average). If that's the case, then this cell is almost all silicon anode, and according to information from Welion's previous launch, all silicon is clearly impractical. If we analyse the battery system's rated voltage of 337V, 4P96S, the voltage of a single cell is 337/96=3.51V, not 3.31V as above, which is probably the 1C discharge voltage, not the 1/3C standard discharge voltage (judging from the side, this cell's multiplier performance may be poor). As it is silicon carbon and high nickel, in the case of an average voltage of 3.51V, guess the available voltage range is 2.5~4.25V, the lower line is not 2.8V because of the higher voltage platform of silicon caused. In addition, analysing the capacity of different battery packs in comparison with the capacity of a single unit, it can be seen that the current CATL uses oversized cells, which only need to be connected in series, without the need for parallel connection, and the battery pack structure is the simplest. CALB cell capacity is halved, requiring two parallel 2P, and the system electrical connection is slightly more complex. As for Welion, due to the limitation of the thickness of the soft pack battery cell (the depth of the aluminium-plastic film is too deep and easy to rupture, the general thickness of the soft pack is not more than 13mm, and the dimensions of Welion's battery cell is 118*359*12mm), the single capacity is only 110Ah, and it must be 4P to satisfy the capacity of the system which is 446Ah, so it can be seen that it will be more complicated to form a group, and it may have a greater impact on the utilization rate of the volume. The system will be more complex and may have a greater impact on the volume utilisation. However, 150kWh is double the capacity of 75kWh and 50% higher than 100kWh because the volume of the whole pack remains unchanged. This is mainly due to the fact that the specific energy of the cell, 360Wh/kg, is just too high, 26% higher than the 285Wh/kg of the Qilin battery. Based on the system specific energy of 260Wh/kg, the mass group efficiency is 72.4%, which is close to the 73.9% of CATL's first generation CTP 100kWh version.

    How to achieve the ultra-high energy density of 360Wh/kg is the core of this 110Ah semi-solid-state-battery cell. Adopting 90% nickel NMC grams capacity play is expected to reach a maximum of about 215mAh/g. Considering the low first-time Coulomb efficiency of silicon carbon anode, we assume that the capacity of cathode will drop to 205mAh/g. Under the rated voltage of 3.51V, the proportion of cathode material should reach 50% in order to achieve the energy density of 360Wh/kg. At present, the mainstream NMC cell its cathode material mass ratio is about 40%, semi-solid-state-battery also added additional solid electrolyte coating (can be coated on the diaphragm or pole piece, refer to the semi-solid-state-battery with high energy density), want to increase to 50% on the basis of 40% is not easy. Need to work together in the following areas:


    1) Improve the cathode surface density and reduce the amount of other auxiliary materials such as copper foil and aluminium foil. At present, the surface density of mainstream NMC cells (double-sided) is around 360~400g/m2, in order to achieve ultra-high specific energy, the surface density of the cathode of Welion may be close to 500g/m2. According to the capacity of 205mAh/g, the surface capacity of single-sided is as high as 5.125mAh/cm2, which is a great improvement with the current 3~4mAh/cm2 of the mainstream power cells. In the case of graphite anode, since the capacity is only 360mAh/g and the compaction density is lower at 1.6g/cm3, the thickness is too thick to match such high surface capacity (~90µm after assembly/>100µm after formation). As a comparison, the first 285Wh/kg Qilin battery anode thickness is about 70µm after formation, and the thickness of CATL cell pole piece is generally controlled below 70µm, and the mainstream cell is generally <60µm. Therefore, it can only be adapted by high-capacity silicon-carbon anode.


    2) Improve the gram capacity of anode material and minimise the mass ratio of anode. In the analysis of Amprius 500Wh/kg silicon-based battery, the theoretical gram capacity of silicon anode is as high as 4200mAh/g, and the actual gram capacity of 3500mAh/g can be achieved (specifically refer to how 500Wh/kg ultra-high-energy batteries are achieved). As the voltage of Welion battery cell has been reduced, the amount of silicon is expected to be more than 20%, and the gram capacity of silicon carbon anode may be 800mAh/g, in which case its single-sided density is only 6.4mg/cm2 (assuming that the N/P is 0), and the corresponding thickness is 40µm under the condition that the compaction density remains unchanged at 1.6g/cm3. However, it is important to note that the expansion of the silicon negative electrode is very large, which is far greater than that of the graphite anode, which is 7%. However, it should be noted that the expansion of silicon anode is very large, much larger than the 7% of graphite anode, and may reach more than 25% expansion, so that the thickness will be more than 50 µm after the formation, and at the level of the battery cell, even after the formation, the battery cell still expands during charging and discharging, and the thickness of the 100% SOC will be more than 6% higher than that of the 10% SOC. Therefore, in order to reduce the impact of expansion, on the one hand, Pack design requires special buffer foam, on the other hand, it also limits the use of SOC range to 5~95%, which is 4% smaller than the current mainstream ternary 3~97%. From the point of view of consumer use, it is definitely the best 0~100%. Welion battery cell only 90% of the available range, for consumers is not cost-effective (after all, they buy the battery in accordance with 100% SOC). But limiting the SOC interval is good for the life of silicon-based batteries, reducing the amount of silicon involved in the reaction, and the expansion will be better (refer to the analysis of Qilin Battery's first high-ratio battery cell)

    3) Reduce the amount of electrolyte: the proportion of electrolyte in the mass of the core is generally between 15~20%, this is a semi-solid-state-battery with electrolyte coating, theoretically more should be reduced by reducing the amount of electrolyte to improve energy density. This can be achieved through high pressure solid, low porosity, reduce the thickness of the pole piece and other ideas. Through preliminary analysis, in the case of the total mass of 1132g of the core, the electrolyte is 180g, and its proportion is 15.9%. And the calculated liquid retention is 1.6g/Ah, which is a very low level. It is much lower than the current mainstream high nickel 2.5g/Ah level, it can be seen that the higher capacity of the grams of silicon carbon anode can be thinner, for reducing the amount of liquid retention is favourable.

    In summary, in order to achieve 360Wh/kg of ultra-high energy density, this Welion cell adopts 90% nickel NMC materials, the gram capacity reaches to 205mAh/g, coupled with 500g/m2 ultra-high double-sided surface density, surface capacity of more than 5mg/cm2, while matching the capacity of up to 800mAh/g of silicon carbon anode, which reduces the amount of anode as well as the amount of electrolyte retaining solution, and the liquid retention coefficient of 1.6g/Ah is 1.6g/Ah in the ultra-low. With the support of ultra-low liquid retention factor of 1.6g/Ah, 50.5% of anode material is achieved. However, there is a problem that the proportion of electrolyte weight is close to 16%, according to the definition generally accepted by the Institute of Physics of the Chinese Academy of Sciences and the industry before, the proportion of liquid in the cell is still in the category of liquid electrolyte in the range of 15~25%. Even if the solute LiPF6 is not taken into account (liquid consists of solute and solvent, the former is solid, the latter is liquid), for 180g of electrolyte, 1mol/L LiPF6 and 1.2g/cm3 density, the solute mass of 22.8g, the solvent mass of 157.2g, accounting for the proportion of the overall mass of the core of about 13.8%, it is basically also a liquid electrolyte. From this level, the addition of solid electrolyte is called semi-solid-battery is still a bit far-fetched (of course, if the liquid mass squeezed out of the battery, the proportion of the battery to do within 10% or no problem).


    Safety: It is worth noting that Mr Li Hong of the Institute of Physics has mentioned that it is not difficult for lithium batteries to achieve a high specific energy by pushing the material, but the key is whether the safety performance meets the requirements for use. This 360Wh/kg cell, its ultra-high nickel cathode as well as high-capacity silicon-carbon anode safety are not good, the former split into low oxygen temperature, the latter SEI is unstable and easy to heat (the impact of which can be referenced to the absolute safety of lithium batteries almost does not exist). Although the so-called in-situ curing technology to protect the body, it is still difficult to pass the GB/T 31485 pinprick test. Need to use similar to the public PV8450-2021 needling method: the use of 1mm diameter steel needle, 2mm / s speed into the centre of the large surface, the depth of 1.2mm (compared with the PV8450-2021, the needling speed from 0.1mm / s to 2mm / s, the depth of 2mm from 2mm to 1.2mm; specific reference to the needling can be returned to the mandatory safety standards). The needling speed has increased from 0.1mm/s to 2mm/s and the depth has decreased from 2mm to 1.2mm. As analysed in the previous article on pinning safety, this test method is to simulate the discharge effect of local short circuit, which is the result of consultation with the host factory on the one hand, and at the same time, it will be easier for many battery cells to pass the test. In addition, due to the use of ultra-high nickel material in this battery cell, the capacity utilisation rate is already very high (expected charging utilisation rate of 88% or more), and the overcharging safety will be a bit better. At the same time, due to the use of solid electrolyte layer, the protection against extrusion will also be enhanced, and extrusion safety is also guaranteed. The hot box test national standard of 130°C is particularly easy to pass, and due to the use of diaphragm coating, it can generally go up to 200°C without shrinkage. Considering that the positive pole material is divided into interpreted oxygen temperature becomes lower, generally around 180 ℃, so this core hot box from 130 ℃, can reach 160 ℃ after sticking to one hour no problem. In addition, external short circuit as well as over discharge are safety tests that are easier to pass. Overall, with the exception of the pinprick, this battery cell performed reasonably well in the safety tests. However, it should be noted that the quality control of such a high-capacity battery cell is crucial, and the expansion of the battery cell during use may damage the aluminium-plastic film, so pack Integration should also give special consideration to the impact of the expansion of the battery cell. The safety of the battery cell after aging has yet to be verified.


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