Separator is one of the important components of lithium battery. It plays the role of isolating positive and negative electrodes, preventing short circuit and providing microporous channel for Li + migration in lithium battery. It can close pores to prevent Li + migration under uncontrolled conditions and ensure the safety of battery. The diaphragm does not directly participate in the reaction of the electrode, but has an important impact on the comprehensive performance of lithium battery.
01
Traditional diaphragm vs coated diaphragm
Traditional polyethylene (PE) and polypropylene (PP) organic diaphragms will have problems of low melting point and insufficient mechanical strength in practical use, and are easy to cause battery short circuit under extreme conditions. Inorganic ceramic materials have attracted much attention because of their high melting point, good chemical stability and good affinity with electrolyte. At present, mainstream battery diaphragm manufacturers including Shanghai Enjie, Xingyuan material and Cangzhou pearl have launched their own coated diaphragms. Figure 1 shows the comparison before and after coating ceramic particles on the base film.
Figure 1 comparison diagram before and after coating ceramic particles on the base film (picture source: Shanghai Enjie)
So, how much does ceramic coated diaphragm improve the performance of battery diaphragm? Can there be specific quantitative experiments to explain it? The answer is yes. In order to facilitate readers’ reading, Xiaobian sorted out the advantages of ceramic coated diaphragm according to the comparative experiments of two different units on composite diaphragm and traditional diaphragm.
02
Improvement of diaphragm performance by ceramic coating
The first set of comparative experimental results comes from Dongguan Zhige Battery Technology Co., Ltd., which focuses on the research and comparison of the performance of the modified diaphragm itself. Table 1 shows the conditions involved in the experimental comparison.
Table 1 experimental conditions of lattice cell
| 东莞市致格电池科技有限公司 | |
| 基膜 | 16um厚PE隔膜（苏州产） |
| 改性粉体 | 25g纳米Al2O3颗粒（日本产，AKP-3000型） |
| 重要设备 | P-5球磨机（德国产）：用于粉体、粘结剂混合球磨 |
| 电解液 | 1.2mol/L 六氟磷酸锂+EC+DEC+EMC（珠海产） |
The physical and chemical properties of the base film and the coating film were compared. The experimental results are shown in Table 2.
Table 2 performance comparison of diaphragm before and after modification
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| PE隔膜 | Al2O3涂布隔膜 | ||||
| 透气率 | 287s/100ml | 303s/100ml | |||
| 孔隙率 | 38.82％ | 40.82％ | |||
| 面密度 | 9.3g/m2 | 15.90g/m2 | |||
| 物理性能方面，透气率和孔隙率变化小，说明陶瓷颗粒没有堵住PE隔膜间隙 | |||||
| 微观形貌 |  |
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| PE隔膜具有一致的孔隙结构，利于离子快速移动；
涂布隔膜可以见到均匀分布的陶瓷颗粒，无明显的堵孔现象 |
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| 热收缩率（130℃） | 纵向 | 横向 | 纵向 | 横向 | |
| 18.0 | 17.8 | 1.1 | 1.1 | ||
| 在骨架支撑作用下，130℃时，PE隔膜纵向和横向上的热收缩率达到18％左右，边缘严重收缩，可能使得电池正负极直接接触，存在热失控危险；
涂布隔膜耐高温作用更明显，这是因为陶瓷颗粒耐高温且隔热，形成刚性的支撑骨架，可明显改善隔膜的热收缩问题 |
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| 浸润性
实验 |
扩散现象 | 不均匀扩散，椭圆扩散 | 均匀扩散，圆形扩散 | ||
| 扩散直径 | 31.4mm | 29.0mm | |||
| 扩散时间 | 26.0s | 24.4s | |||
| 消失时间 | 53.6s | 43.4s | |||
| 浸润性实验是将电解液滴在隔膜上进行的，改性隔膜能够明显缩短电解液扩散时间，这是因为Al2O3陶瓷颗粒极性较高，与电解液亲和性更好 | |||||
03
The Institute of nuclear energy and new energy technology of Tsinghua University also studied the performance of the diaphragm. The research focused on the performance of the modified diaphragm after it was assembled into the battery. The self-made highly dispersed polymethylmethacrylate (PMMA) coated Al2O3 nano hybrid material (abbreviated as Al2O3@PMMA Nano hybrid materials). On the basis of self-made coating material, PP was prepared/ Al2O3@PMMA /Important basic materials used for PVDF-HFP composite diaphragm are shown in Table 3.
Table 3 important materials involved in Tsinghua University Experiment
| 清华大学核能与新能源技术研究院 | |
| 基膜 | 25um厚PP基Celgard2325膜（美国产） |
| 改性粉体 | 1g自制的Al2O3@PMMA纳米杂化材料 |
| 其它基材 | N,N-二甲基甲酰胺（DMF，国药集团），PVDF-HFP（Aldrich公司，电池级） |
Table 4 shows the performance comparison of polymer diaphragm and composite diaphragm by Tsinghua University.
Table 4 performance comparison between composite diaphragm and traditional diaphragm
| 清华大学核能与新能源技术研究院 | ||
| PP/PVDF-HFP隔膜 | 复合隔膜 | |
| 微观形貌 |
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| PP/PVDF-HFP隔膜表面孔隙较小，孔径较小；
添加Al2O3@PMMA杂化材料后，形成网状微孔，孔径较大 |
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| 保液率 |  |
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| 复合隔膜保液能力比PP隔膜和PP/PVDF-HFP隔膜强，
这是因为Al2O3@PMMA纳米杂化材料的亲液力更强 |
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| 循环性能 |  |
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| 当100次循环时，复合隔膜、PP隔膜和PP/PVDF-HFP隔膜组装的电池，容量保持率分别为89.8％、86.6％和78.4％，说明Al2O3纳米粒子能提高电解质的离子电导率以及与电极的界面相容性，利于Li+的传输 | ||
| 倍率性能 |  |
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| PP/PVDF-HFP隔膜组装的电池在8C放电后比容量降至40mAh/g，而复合隔膜和PP隔膜保持较高，说明Al2O3粒子的加入，提高了电池倍率性能 | ||
In general, ceramic particles (mainly Al2O3 at present) can be applied to the coating of traditional diaphragm. Its advantages are reflected in the improvement of physical and chemical properties of diaphragm itself: thermal shrinkage resistance and wettability (the experimental results of grid cell). After being assembled into the battery, it can improve the cycle performance and magnification performance of the battery (the experimental results of Tsinghua University).
In the past 2019, the reshuffle of lithium battery diaphragm industry accelerated, and the biggest winner should belong to Shanghai Enjie. Its annual net profit in 19 years is expected to reach more than 770 million yuan, with a year-on-year increase of more than 50% and a market value of 40 billion yuan. Among them, wet diaphragm production and sales are booming. It is worth noting that Shanghai Enjie concentrated its main capital and technology in the enterprise of wet diaphragm at the beginning of its establishment in 2010, and successfully applied wet + single-sided / double-sided ceramic coated diaphragm to lithium battery in 2014.
In the future, with the increase of energy density of lithium battery, the requirements for diaphragm will also increase. Before the full arrival of solid electrolyte, wet + coating is the best solution for lithium battery diaphragm.
reference
Safety performance of Al2O3 ceramic modified diaphragm, Dongguan Zhige Battery Technology Co., Ltd., Yang Heshan; Shenzhen Qingxin Power Research Institute, Shi yuliei.
Performance of highly dispersed nano Al2O3 modified composite electrolyte membrane, Institute of nuclear energy and new energy technology, Tsinghua University, Zhou Ranran, he Xiangming, Shang Yuming; Yanshan University, Li Jianjun.