-
研究对象为锂离子电池负极极片材料,该材料为“三明治”结构,由铜箔及其上下表面涂覆的等厚度石墨层组成,材料总厚度为100μm,其中铜箔厚度为10μm,材料结构如图 1所示。切割过程中,激光束辐照至负极材料上表面,上层石墨材料吸收能量后温度迅速上升至气化点,同时热量传导至中间金属层与下层石墨,产生材料的去除[14]。通过扫描振镜控制激光光斑位置,调整其沿切割路径连续移动,由此完成切割过程[15]。
激光切割过程中,光斑所在区域的温度随时间急剧变化,激光切割温度场求解属于非线性的瞬态导热问题。温度场分析基于能量控制方程,利用傅里叶定律和能量守恒定律建立导热微分方程求解,求得内部各点的热流密度矢量,获得材料体内的温度场。为简化计算条件,提出3点假设[16]:(1)材料是各向同性的连续介质;(2)考虑材料物性参量(导热率、比热以及密度大小)的温度依赖性;(3)忽略石墨层与铜箔间的热损耗以及热阻的影响。
-
在激光与材料的作用过程中,温度t随时间τ变化,属于非稳态导热,直角坐标系中3维非稳态导热微分方程的一般形式为[17]:
$ \rho c \frac{\partial}{\partial \tau}=\frac{\partial}{\partial x}\left(\lambda \frac{\partial t}{\partial x}\right)+\frac{\partial}{\partial y}\left(\lambda \frac{\partial t}{\partial y}\right)+\frac{\partial}{\partial z}\left(\lambda \frac{\partial t}{\partial z}\right)+q' $
(1) 式中,ρ(kg/m3)为材料的密度,c为材料的比热容,λ[W/(m·K)]为材料的热导率,q′为内热源。根据本节中对模型材料常物性的假设,对应该模型的导热过程,可将导热微分方程简化为物性参量为常数、无内热源的傅里叶方程,即:
$ \frac{\partial t}{\partial \tau}=a\left(\frac{\partial^{2} t}{\partial x^{2}}+\frac{\partial^{2} t}{\partial y^{2}}+\frac{\partial^{2} t}{\partial z^{2}}\right) $
(2) 式中,a=λ/(ρc), x, y, z为空间坐标。
-
初始条件下,设置初始温度t0=283.97K。由于材料在激光烧蚀过程中温度急剧升高,其温度变化还需考虑光斑辐照区域与周围介质间的对流换热与辐射换热。因此,在激光热源辐照区域内,激光热源以热流密度形式加载在材料上表面,除此之外,所有表面均设置为对流换热边界与辐射换热边界,热传递量q的对流边界条件[18]可表示为:
$ q=h \Delta t $
(3) 式中,Δt指极片材料表面与空气流体间的温度差,h表示表面换热系数。
材料的辐射边界条件,即极片与周围空气间的热传递量q可以表示为:
$ q = \varepsilon \sigma S\left( {{t_{\rm{m}}}^4 - {t_{\rm{a}}}^4} \right) $
(4) 式中,ε为辐射系数,取ε=0.7,S表示极片材料的面积,tm为材料温度,ta为周围空气的温度,初始条件下,tm=ta=283.97K, 斯忒藩-玻尔兹曼常数σ=5.67×10-8W(m2·K4)。
激光切割过程中,温度的高低、温度梯度的大小取决于板材的受热面积、板材吸收的热流密度和加热时间[19-20],除(3)式、(4)式所表达的对流边界条件和辐射边界条件外,将激光热源视为施加于材料表面的一类边界条件。本文中的研究对象为厚度约100μm的负极极片材料,高斯热源模型可描述其切割温度场的分布。高斯热源模型是一种面热源,切割过程中通过激光热源的加载为材料去除提供热能,其中表面有效加载区域为光斑。加热区域内热流密度近似为高斯函数分布,热源S0的数学表达式[20]如下:
$ \begin{array}{c} {S_0} = {I_0}\left( {1 - {\gamma _{\rm s}}} \right)\exp \left\{ {\left[ {\left( { - {{\left( {x - {x_0}} \right)}^2} + } \right.} \right.} \right.\\ \left. {\left. {\left. {{{\left( {y - {y_0}} \right)}^2}} \right)/r_0^2} \right]} \right\} \end{array} $
(5) 式中,γs为材料表面的反射系数,δ为材料沿厚度方向的吸收率,(x0, y0)为光斑中心坐标,r0为光斑半径,I0为光斑中心强度, P为激光功率。根据高斯分布理论,光斑中心强度可表示为:
$ {I_0} = \frac{{2P/\left( {{\rm{ \mathsf{ π} }}r_0^2} \right)}}{{1 - {{\rm{e}}^{ - 2}}}} $
(6) 入射激光波长为1065nm,该条件下石墨和铜的材料特性参量如表 1所示。为准确地模拟复合材料的特性,表面吸收性质与水平面传播方向上的热传导表现为石墨材料的性质。
Table 1. Material properties of copper and graphite
properties copper graphite density/(kg·m-3) 8.96×103 1.73×103 thermal conductivity/(W·m-1·K-1) 317 18.1 specific heat/(J·kg-1·K-1) 385 2092.48 surface reflectance 0.97267 0.16815 melting temperature/K 1358 3652 boiling temperature/K 2835 4800 -
仿真过程为3-D瞬态热分析,采用ANSYS solid 90单元,适用于3-D的稳态或瞬态热分析问题。如图 2所示,该单元为3-D 20个节点的热实体,每节点具有一个温度自由度。采用扫掠方式划分六面体网格,按照载荷特征,根据网格与切缝的距离决定划分精度,由外向内逐渐提高,对切缝部分的网格进一步细化,该模型一共具有5252个节点,3600个单元[21]。考虑到切缝两侧的对称性,故只取切缝一侧进行求解。由此建立材料3-D模型,其尺寸为1mm×0.5mm×0.1mm。
激光切割锂电池负极极片复合材料的数值模拟
Numerical simulation of laser cutting composite anodes for lithium-ion batteries
-
摘要: 为了探究负极极片复合材料的切割性质, 采用有限元模型, 对激光切割锂离子电池负极极片复合材料温度场进行数值模拟计算。由温度场分布取得了负极切缝宽度与切缝深度的尺寸大小, 从中研究激光功率、切割速率和光斑半径对负极表层材料切缝宽度与切缝深度的影响。结果表明, 负极表层切缝宽度随激光功率和光斑半径的增加而增大, 随切割速率增大而减小; 切缝深度随激光功率增加而增大, 随切割速率和光斑半径增大而减小。切割至中间铜箔后, 切缝深度变化速率趋缓, 负极材料的复合结构对切缝深度存在明显影响; 在功率为170W、光斑半径为47μm、切割速率变化至600mm/s左右时, 效果最为明显, 切割深度在该参量下达到60μm, 且突破这一阈值后增长速率得到明显提升直至极片完全切断。这一结果可为激光应用于锂离子电池极片复合材料切割提供参考。Abstract: In order to investigate cutting characteristics of negative plate composites, the temperature field of negative plate composite material of the lithium ion battery by laser cutting was numerically simulated based on finite element model. The size of the width and depth of the negative electrode slit was obtained from the temperature field distribution. The effect of laser power, cutting speed and spot radius on the slit width and depth of negative surface material was studied. The results show that, the width of the slit on the surface of the negative electrode increases with the increase of laser power and the radius of the spot. It decreases with the increase of cutting speed. The slit depth increases with the increase of laser power. It decreases with the increase of cutting speed and spot radius. After cutting to the middle copper foil, the change rate of the slit depth is slowing down. The composite structure of the negative electrode material has a significant effect on the depth of the slit. When the power is 170W, the radius of the spot is 47μm, and the cutting speed varies to about 600mm/s, the effect is most obvious. The cutting depth reaches 60μm under this parameter. After breaking through this threshold, the growth rate is increased significantly until the polar plate is completely cut off. The results can provide a reference for laser cutting of lithium ion battery plate composite material.
-
Key words:
- laser technique /
- laser cutting /
- temperature simulation /
- electrode anode
-
Table 1. Material properties of copper and graphite
properties copper graphite density/(kg·m-3) 8.96×103 1.73×103 thermal conductivity/(W·m-1·K-1) 317 18.1 specific heat/(J·kg-1·K-1) 385 2092.48 surface reflectance 0.97267 0.16815 melting temperature/K 1358 3652 boiling temperature/K 2835 4800 -
[1] PETERSON S B, WHITACRE J F, APT J. Net air emissions from electric vehicles: the effect of carbon price and charging strategies[J]. Environmental Science & Technology, 2011, 45(5):1792-1797. [2] LIAO X, YU J, GAO L.Electrochemical study on lithium iron phosphate/hard carbon lithium-ion batteries[J]. Journal of Solid State Electrochemistry, 2012, 16(2):423-428. doi: 10.1007/s10008-011-1387-7 [3] KOJIMA T, ISHIZU T, HORIBA T, et al. Development of lithium-ion battery for fuel cell hybrid electric vehicle application[J]. Journal of Power Sources, 2009, 189(1):859-863. doi: 10.1016/j.jpowsour.2008.10.082 [4] SCROSATI B, JVRGEN G. Lithium batteries: Status, prospects and future[J]. Journal of Power Sources, 2010, 195(9):2419-2430. doi: 10.1016/j.jpowsour.2009.11.048 [5] YANG L X, PENG X F, WANG B X. Numerical modeling and experimental investigation on the characteristics of molten pool during laser processing[J]. International Journal of Heat and Mass Transfer, 2001, 44(23):4465-4473. doi: 10.1016/S0017-9310(01)00086-2 [6] CENG W M S M. Laser material processing[M]. London, UK:Springer, 2010:21-27. [7] LEE D, PATWA R, HERFURTH H, et al.High speed remote laser cutting of electrodes for lithium-ion batteries: Anode [J].Journal of Power Sources, 2013, 240:368-380. doi: 10.1016/j.jpowsour.2012.10.096 [8] PFLEGING W. A review of laser electrode processing for development and manufacturing of lithium-ion batteries[J]. Nanophotonics, 2017, 7(3):549-573. [9] SCHMIEDER B. Laser cutting of graphite anodes for automotive lithium-ion secondary batteries: Investigations in the edge geometry and heat-affected zone[C]// Laser-based Micro- & Nanopackaging & A-ssembly Ⅵ. New York, USA: International Society for Optics and Photonics, 2012: 31-56. [10] KRONTHALER M R, SCHLOEGL F, KURFER J, et al. Laser cutting in the production of lithium ion cells[J]. Physics Procedia, 2012, 39:213-224. doi: 10.1016/j.phpro.2012.10.032 [11] DEMIR A G, PREVITALI B. Remote cutting of Li-ion battery electrodes with infrared and green ns-pulsed fibre lasers[J]. The International Journal of Advanced Manufacturing Technology, 2014, 75(9/12):1557-1568. [12] PFLEGING W. Laser cutting of graphite anodes for automotive lithium-ion secondary batteries: Investigations in the edge geometry and heat-affected zone[J]. Proceedings of the SPIE, 2012, 8244:23-27. [13] LEE D, PATWA R, HERFURTH H, et al. Parameter optimization for high speed remote laser cutting of electrodes for lithium-ion batte-ries[J]. Journal of Laser Applications, 2016, 28(2):022006. doi: 10.2351/1.4942044 [14] KURFER J, WESTERMEIER M, TAMMER C, et al. Production of large-area lithium-ion cells—Preconditioning, cell stacking and quality assurance[J]. CIRP Annals-manufacturing Technology, 2012, 61(1):21-32. doi: 10.1016/j.cirp.2012.03.121 [15] WANG Y Sh, YANG X Ch, LIU Y J. Temperature field of laser scanning line facula[J]. Chinese Journal of Lasers, 2006, 33(7):981- 986(in Chinese). [16] LIU X X, HUANG R, YAO G, et al. Numerical simulation of the temperature field of laser butt welding of titanium alloy sheet[J]. Laser Technology, 2013, 37(5): 700-704(in Chinese). [17] MA J, ZHAO Y, ZHOU F Y, et al. Study on temperature field of materials irradiated by pulse laser based on FEM [J]. Laser & Infrared, 2015, 45 (1): 27-31(in Chinese). [18] YILBAS B S, ARIF A F M, ALEEM B J A. Laser cutting of sharp edge: Thermal stress analysis[J]. Optics & Lasers in Engineering, 2010, 48(1):10-19. [19] DENG D Sh, LIU J H, HU X Y, et al. Steady-state conditions for oxygen assisted lasox laser cutting of thick plates[J]. Laser Technology, 2003, 27(3):178-181(in Chinese). [20] NYON K Y, MOKHTAR M, ABDUL-RAHMAN R. Finite element analysis of laser inert gas cutting on Inconel 718[J]. International Journal of Advanced Manufacturing Technology, 2012, 60(9/12):995-1007. [21] YANG H L, JIN X Zh, XIU T F, et al. Numerical simulation of fiber laser welding of steel /aluminum dissimilar metals[J]. Laser Technology, 2016, 40(4):606-609(in Chinese).