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What are the factors that affect the battery charge?

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What are the factors that affect the battery charge?

China Energy Storage Network News:

Lithium batteries are called "rocking chair type" batteries. The charged ions move between the positive and negative electrodes to realize charge transfer, to supply power to external circuits or to charge from external power sources.
During the specific charging process, the external voltage is applied to the two poles of the battery, lithium ions are deintercalated from the positive electrode material, enter the electrolyte, and at the same time, excess electrons are generated to pass through the positive current collector, and move to the negative electrode through an external circuit; lithium ions are in the electrolyte. The positive electrode moves toward the negative electrode and passes through the separator to reach the negative electrode; the SEI film passing through the surface of the negative electrode is embedded in the negative graphite layered structure and bonded to the electron.
During the entire ion and electron operation, the battery structure that affects charge transfer, whether electrochemical or physical, will have an impact on fast charge performance.
Fast charge requirements for various parts of the battery
For the battery, if you want to improve the power performance, you need to work hard in all aspects of the battery, including the positive electrode, negative electrode, electrolyte, diaphragm and structural design.

 

positive electrode
In fact, almost all kinds of cathode materials can be used to make fast-fill batteries. The main performances required to be guaranteed include conductance (reduction of internal resistance), diffusion (guaranteed reaction kinetics), longevity (no need to explain), and safety (not required). Explain), proper processing performance (the specific surface area can not be too large, reduce side reactions, for safety services).
Of course, the problems to be solved for each specific material may vary, but our common cathode materials can be optimized through a series of optimizations, but different materials are also different:
A. Lithium iron phosphate may be more focused on solving problems of conductance and low temperature. Carbon coating, moderate nanocrystallization (note that it is moderate, definitely not as fine as the simple logic), the formation of ionic conductors on the surface of the particles is the most typical strategy.
B, the ternary material itself has a good conductance, but its reactivity is too high, so the ternary material has little work of nanocrystallization (nanocrystallization is not an antidote to the performance improvement of the metallurgical material, especially in the field of batteries. There are sometimes many reactions in the system. More attention is paid to safety and inhibition (and electrolyte) side effects. After all, the main goal of ternary materials is safety. The recent battery safety accidents are also frequent. Put forward higher requirements.
C, lithium manganate is more important for life, there are a lot of fast-charge batteries of lithium manganate on the market.
negative electrode
When the lithium ion battery is charged, lithium migrates to the negative electrode. The excessively high potential caused by the fast charge and high current will cause the negative electrode potential to be more negative. At this time, the pressure of the negative electrode rapidly accepting lithium will become larger, and the tendency to generate lithium dendrites will become larger. Therefore, the negative electrode must not only satisfy the lithium diffusion during fast charging. The kinetic requirements, but also to solve the safety problems caused by the increased tendency of lithium dendrite formation, so the main technical difficulty of the fast charging core is the insertion of lithium ions in the negative electrode.
A. At present, the dominant anode material in the market is still graphite (accounting for about 90% of the market share), the root cause is none--cheap, and the comprehensive processing performance and energy density of graphite are excellent, and the disadvantages are relatively few. . Of course, graphite anodes also have problems. The surface is sensitive to electrolytes, and the lithium intercalation reaction has strong directionality. Therefore, it is mainly necessary to work hard to carry out graphite surface treatment, improve its structural stability, and promote the diffusion of lithium ions on the substrate. direction.
B. Hard carbon and soft carbon materials have also developed in recent years: hard carbon materials have high lithium insertion potential, micropores in the materials, and good reaction kinetics; and soft carbon materials have good compatibility with electrolytes, MCMB The materials are also very representative, but the hard and soft carbon materials are generally low in efficiency and high in cost (and Imagine that graphite is as cheap as I hope from an industrial point of view), so the amount is far less than graphite, and more used in some specialties. On the battery.
C, how about lithium titanate? To put it simply: lithium titanate has the advantages of high power density, safer, and obvious disadvantages. The energy density is very low, and the calculation cost is high according to Wh. Therefore, the viewpoint of lithium titanate battery is a useful technology that is advantageous in certain occasions, but it is not suitable for many occasions where the cost and cruising range are high.
D, silicon anode material is an important development direction, Panasonic's new 18650 battery has begun commercial process for such materials. But how to achieve a balance between the pursuit of performance in nanotechnology and the battery industry's general micron-scale requirements for materials is still a challenging task.

Diaphragm
For power batteries, high current operation provides higher requirements for safety and longevity. Diaphragm coating technology is inseparable. Ceramic coated membranes are rapidly being pushed away due to their high safety and the ability to consume impurities in the electrolyte. Especially for the safety of ternary batteries, the safety effect is particularly remarkable.
The main system currently used in ceramic diaphragms is to coat alumina particles on the surface of conventional diaphragms. A relatively novel approach is to coat solid electrolyte fibers on the membrane. Such membranes have lower internal resistance and mechanical support for the membrane. Excellent, and it has a lower tendency to block the diaphragm hole during service.
After the coating, the separator has good stability. Even if the temperature is relatively high, it is not easy to shrink and deform, resulting in short circuit. Jiangsu Qingtao Energy Co., Ltd., technical support of the Academic Researcher of Tsinghua University School of Materials, has some representative aspects in this respect. Work, the diaphragm is shown below.
Electrolyte
The electrolyte has a great influence on the performance of a fast-charged lithium ion battery. To ensure the stability and safety of the battery under fast charge and high current, the electrolyte should meet the following characteristics: A) can not be decomposed, B) the conductivity is high, C) is inert to the positive and negative materials, can not React or dissolve.
If these requirements are to be met, the key is to use additives and functional electrolytes. For example, the safety of ternary fast-charged batteries is greatly affected by it. It is necessary to add various anti-high temperature, flame-retardant and anti-overcharged additives to protect them to a certain extent. The problem of the old lithium titanate battery, the high temperature flatulence, also depends on the high temperature functional electrolyte.
Battery structure design
A typical optimization strategy is the stacked VS winding type. The electrodes of the laminated battery are equivalent to a parallel relationship, and the winding type is equivalent to a series connection. Therefore, the internal resistance of the former is much smaller, and it is more suitable for the power type. occasion.
In addition, you can work hard on the number of poles to solve internal resistance and heat dissipation problems. In addition, the use of high-conductivity electrode materials, the use of more conductive agents, and the coating of thinner electrodes are also strategies that can be considered.
In short, factors affecting the internal charge movement of the battery and the rate of embedding the electrode cavity will affect the rapid charging capability of the lithium battery.

 

中国储能网讯:锂电池被称为“摇椅型”电池,带电离子在正负极之间运动,实现电荷转移,给外部电路供电或者从外部电源充电。

具体的充电过程中,外电压加载在电池的两极,锂离子从正极材料中脱嵌,进入电解液中,同时产生多余电子通过正极集流体,经外部电路向负极运动;锂离子在电解液中从正极向负极运动,穿过隔膜到达负极;经过负极表面的SEI膜嵌入到负极石墨层状结构中,并与电子结合。

在整个离子和电子的运行过程中,对电荷转移产生影响的电池结构,无论电化学的还是物理的,都将对快速充电性能产生影响。

快充对电池各部分的要求

对于电池来说,如果要提升功率性能,需要在电池整体的各个环节中都下功夫,主要包括正极、负极、电解液、隔膜和结构设计等。

正极

实际上,各种正极材料几乎都可以用来制造快充型电池,主要需要保证的性能包括电导(减少内阻)、扩散(保证反应动力学)、寿命(不需要解释)、安全(不需要解释)、适当的加工性能(比表面积不可太大,减少副反应,为安全服务)。

当然,对于每种具体材料要解决的问题可能有所差异,但是我们一般常见的正极材料都可以通过一系列的优化来满足这些要求,但是不同材料也有所区别:

A、磷酸铁锂可能更侧重于解决电导、低温方面的问题。进行碳包覆,适度纳米化(注意,是适度,绝对不是越细越好的简单逻辑),在颗粒表面处理形成离子导体都是最为典型的策略。

B、三元材料本身电导已经比较好,但是其反应活性太高,因此三元材料少有进行纳米化的工作(纳米化可不是什么万金油式的材料性能提升的解药,尤其是在电池领域中有时还有好多反作用),更多在注重安全性和抑制(与电解液的)副反应,毕竟目前三元材料的一大命门就在于安全,近来的电池安全事故频发也对此方面提出了更高的要求。

C、锰酸锂是则对于寿命更为看重,目前市面上也有不少锰酸锂系的快充电池。

负极

锂离子电池充电的时候,锂向负极迁移。而快充大电流带来的过高电位会导致负极电位更负,此时负极迅速接纳锂的压力会变大,生成锂枝晶的倾向会变大,因此快充时负极不仅要满足锂扩散的动力学要求,更要解决锂枝晶生成倾向加剧带来的安全性问题,所以快充电芯实际上主要的技术难点为锂离子在负极的嵌入。

A、目前市场上占有统治地位的负极材料仍然是石墨(占市场份额的90%左右),根本原因无他——便宜,以及石墨综合的加工性能、能量密度方面都比较优秀,缺点相对较少。石墨负极当然也有问题,其表面对于电解液较为敏感,锂的嵌入反应带有强的方向性,因此进行石墨表面处理,提高其结构稳定性,促进锂离子在基上的扩散是主要需要努力的方向。

B、硬碳和软碳类材料近年来也有不少的发展:硬碳材料嵌锂电位高,材料中有微孔因此反应动力学性能良好;而软碳材料与电解液相容性好,MCMB材料也很有代表性,只是硬软碳材料普遍效率偏低,成本较高(而且想像石墨一样便宜恐怕从工业角度上看希望不大),因此目前用量远不及石墨,更多用在一些特种电池上。

C、钛酸锂如何?简单说一下:钛酸锂的优点是功率密度高,较安全,缺点也明显,能量密度很低,按Wh计算成本很高。因此对于钛酸锂电池的观点是一种有用的在特定场合下有优势的技术,但是对于很多对成本、续航里程要求较高的场合并不太适用。

D、硅负极材料是重要的发展方向,松下的新型18650电池已经开始了对此类材料的商用进程。但是如何在纳米化追求性能与电池工业对于材料的一般微米级的要求方面达到一个平衡,仍是比较有挑战性的工作。

隔膜

对于功率型电池,大电流工作对其安全、寿命上提供了更高的要求。隔膜涂层技术是绕不开的,陶瓷涂层隔膜因为其高安全、可以消耗电解液中杂质等特性正在迅速推开,尤其对于三元电池安全性的提升效果格外显著。

陶瓷隔膜目前主要使用的体系是把氧化铝颗粒涂布在传统隔膜表面,比较新颖的做法是将固态电解质纤维涂在隔膜上,这样的隔膜的内阻更低,纤维对于隔膜的力学支撑效果更优,而且在服役过程中其堵塞隔膜孔的倾向更低。

涂层以后的隔膜,稳定性好,即使温度比较高,也不容易收缩变形导致短路,清华大学材料学院南策文院士课题组技术支持的江苏清陶能源公司在此方面就有一些代表性的工作,隔膜如下图所示。

电解液

电解液对于快充锂离子电池的性能影响很大。要保证电池在快充大电流下的稳定和安全性,此时电解液要满足以下几个特性:A)不能分解,B)导电率要高,C)对正负极材料是惰性的,不能反应或溶解。

如果要达到这几个要求,关键要用到添加剂和功能电解质。比如三元快充电池的安全受其影响很大,必须向其中加入各种抗高温类、阻燃类、防过充电类的添加剂保护,才能一定程度上提高其安全性。而钛酸锂电池的老大难问题,高温胀气,也得靠高温功能型电解液改善。

电池结构设计

典型的一个优化策略就是叠层式VS卷绕式,叠层式电池的电极之间相当于是并联关系,卷绕式则相当于是串联,因此前者内阻要小的多,更适合用于功率型场合。

另外也可以在极耳数目上下功夫,解决内阻和散热问题。此外使用高电导的电极材料、使用更多的导电剂、涂布更薄的电极也都是可以考虑的策略。

总之,影响电池内部电荷移动和嵌入电极孔穴速率的因素,都会影响锂电池快速充电能力。

Pub Time : 2018-07-23 17:28:05 >> News list
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