Overdischarge-induced evolution of Cu dendrites and degradation of mechanical properties in lithium-ion batteries

The degradation of mechanical properties of overdischarge battery materials manifests as a significant effect on the energy density,safety,and cycle life of the batteries.However,establishing the correlation between depth of overdischarge and mechanical properties is still a significant challenge.Studying the correlation between depth of overdischarge and mechanical properties is of great significance to improving the energy density and the ability to resist abuse of the batteries.In this paper,the mechanical properties of the battery materials during the whole process of overdischarge from discharge to complete failure were studied.The effects of depth of overdischarge on the elastic modulus and hardness of the cathode of the battery,the tensile strength and the thermal shrinkage rate of the separator,and the performance of binder were investigated.The precipitation of Cu dendrites on the separator and cathode after dissolution of anode copper foil is a key factor affecting the performance of battery materials.The Cu dendrites attached to the cathode penetrate the separator,causing irreversible damage to the coating and base film of the separator,which leads to a sharp decline in the tensile strength,thermal shrinkage rate and other properties of the separator.In addition,the Cu dendrites wrapping the cathode active particles reduce the adhesion of the active particles binder.Meanwhile,the active particles are damaged,resulting in a significant decrease in the elastic modulus and hardness of the cathode.

超薄浮法玻璃下表面微细缺陷的表征

利用场发射电子扫描电镜和原子力显微镜对浮法玻璃下表面微细缺陷的形貌进行二维和三维表征,发现表面微细缺陷是由许许多多的微米级的微小划伤、划痕、压痕机附着的微小颗粒组成,单一的缺陷长度为5~20μm,宽度在1μm以下,缺陷纵深5~20 nm。

纳米聚集氧化硅固定磨料抛光布的抛光特性

针对传统磨料的固定磨料抛光布容易在加工表面产生划伤,以及材料去除效率低等问题,提出了采用微米级球形聚集氧化硅粒子的固定磨料抛光布。将纳米聚集氧化硅粒子添加到抛光布中,用pH为10.5的碱性水溶液替代传统的抛光液,进行了Si基板的的抛光加工试验。与传统采用不规则形状天然氧化硅及球形熔融氧化硅固定磨料抛光布进行了比较,得到了纳米聚集氧化硅的固定磨料抛光布的加工特性,并讨论了它的基本参数对加工特性的影响。实验得到了与现行纳米抛光液（重量百分比为3%,pH=10.5）相同的材料去除率,加工表面粗糙度降低了约30%。与传统不规则形状天然氧化硅磨料抛光布相比,纳米聚集氧化硅抛光布的磨料为球形,弹性系数仅为其1.4%-60%,因此不易划伤抛光表面。与熔融氧化硅抛光布相比,纳米聚集氧化硅抛光布在pH为10.5的碱性水溶液中磨料表面可吸附的[-OH]离子提高了25倍,使得液相化学去除作用增大至去除率的70%以上。另外,随着纳米聚集氧化硅的微米粒径的增大,固定磨料抛光布的纳米级加工表面粗糙度几乎不变,但对前加工面表面粗糙度的去除能力明显增大,表现出微米粒径效应。

基于单颗磨粒切削的硅片加工破碎损伤

为了探究硅片器件精密磨削加工破碎的损伤规律与演变机制,开展了单颗金刚石磨粒切削单晶硅片的微米划痕实验,分析了硅片边缘有无胶粘包裹作用两种条件下的划痕入口、内部与出口三个区段的破碎损伤形貌特征,并建立了声发射强度、磨削力、切削深度、摩擦系数与破碎损伤之间的内在密切关联。单晶硅破碎损伤随着加载压力或切入深度的增大而越加严重,伴随释放的声发射信号强度增大。单晶硅内部破碎发生的临界阈值条件:载荷约80 mN,切入深度约2μm,声发射强度约8%。胶粘包裹对单晶硅片边缘的增韧效果显著,边缘崩碎发生临界阈值条件为:载荷约800 mN,切入深度约6μm,声发射强度约55%。

基于微米划痕实验的单晶硅去除机制研究

通过微米划痕实验研究单晶硅在微米尺度下的材料变形和去除特性,分别从划痕轮廓、划痕力以及声发射信号方面分析划痕速度对于单晶硅材料去除方式的影响规律。实验发现:随着划痕速度的提高,单晶硅脆塑转变和脆性破坏阶段的划痕深度和划痕力曲线波动程度减弱,弹塑性去除及脆塑转变的区域范围增加;弹塑性变形和脆塑转变阶段没有声发射信号,声发射信号首次出现在脆性去除阶段起始点,其所对应的载荷随着划痕速度的提高而增大;并且单晶硅弹塑性阶段的划痕力波动程度和增长速率低于脆塑转变和脆性去除阶段的划痕力波动程度和增长速率。在微米尺度下单晶硅塑性去除的深度在50 nm以内,微米尺度下单晶硅的去除方式主要为脆性去除。结果表明,提高划痕速度有利于减小单晶硅脆性去除的范围,从而增大塑性去除和脆塑转变的加工范围。