Basic knowledge of lithium battery

Basic knowledge of lithium battery

Objective record

One 、 front said
Two 、 Classification and performance index of lithium ion batteries
3、 ... and 、 The advantages and disadvantages of lithium-ion batteries
3.1 Advantages of lithium ion batteries
3.2 Disadvantages of lithium ion batteries
Four 、 How lithium ion batteries work
4.1 Working principle of lithium ion battery cell
4.2 Working principle of protection circuit
5、 ... and 、 The development direction of lithium ion batteries

One 、 front said

 since  1958  In, a graduate student at the University of California proposed lithium 、 After the assumption of active metals such as sodium as battery cathode , People began to study lithium batteries . When lithium electrode is replaced by carbon material , That is, the industrial revolution of lithium-ion batteries began . The research of lithium-ion battery began from  1990  Japan  Nagoura  They developed petroleum coke as negative electrode , Lithium ion battery with lithium cobalt oxide as cathode ; Japan in the same year  Sony  And Canada Moli  The two major battery companies announced that they would launch lithium-ion batteries with carbon as the negative pole . Compared with other rechargeable batteries , Lithium ion batteries have high voltage 、 Higher than energy 、 Long charge and discharge life 、 No memory effect 、 Little pollution to the environment 、 Quick charge 、 Low self discharge rate . As an important chemical battery , Lithium ion batteries are made by mobile phones 、 laptop 、 Batteries and submarines for digital cameras and portable small appliances 、 space flight 、 Batteries used in aviation , Gradually move towards the field of electric vehicles . With the increasingly serious global energy and environmental problems , Vehicles have switched to energy storage batteries as the main power source , Lithium ion batteries are considered high capacity 、 Ideal for high-power batteries .

Two 、 Classification and performance index of lithium ion batteries
Lithium ion batteries can be used in various fields , therefore , Its types are also diverse . According to the shape , At present, there are three main types of lithium-ion batteries on the market , Button type 、 Square and cylindrical , As shown in the figure below ：

Cylindrical models use 5 Digit representation , The first two digits indicate the diameter , Third 、 Four digits indicate height .
for example ：18650 Type a battery , Indicates that its diameter is 18mm, The height is 65mm. Square models use 6 Digit representation , The first two digits are the thickness of the battery , The middle two digits are the width of the battery , The last two digits are the length of the battery , for example 083448 type , Indicates that the thickness is 8mm, Width is 34mm, The length is 48mm.

   According to the electrolyte form of lithium-ion battery , Lithium ion batteries include liquid lithium ion batteries and solid-state batteries （ Or dry ） There are two kinds of lithium-ion batteries . Solid state lithium ion battery is commonly known as polymer lithium ion battery , It is a new generation battery developed on the basis of liquid lithium ion battery , It has better safety performance than liquid lithium-ion battery , Liquid lithium-ion battery is commonly known as lithium-ion battery .


   Battery dimensions 、 Weight is an important indicator of lithium-ion batteries , Directly affect the characteristics of the battery . The electrochemical characteristics of lithium-ion batteries mainly include the following aspects ：


   Rated voltage ： The rated voltage of commercial lithium-ion batteries is generally  3.7V, The voltage range during operation is generally  4.2V～3.0V, There are also cases where the lower limit termination voltage is set to other values , Such as  3.1V.
    Rated capacity ： Means in accordance with  0.2C  The capacity obtained when the constant current is discharged to the termination voltage .
   1C  Capacity ： Means in accordance with  1C  Capacity obtained from constant current discharge to termination voltage .1C  The capacity is generally smaller than the rated capacity , The smaller the difference, the better the current characteristics of the battery , The stronger the load capacity .
    High and low temperature performance ： The high temperature of lithium-ion battery can reach +55℃, Low temperature can reach -20℃. Under this ambient temperature range , The battery capacity can reach the rated capacity  70% above .
    Charge holding capacity ： After the battery is fully charged, open the circuit and put it aside  28  God , And then according to  0.2C  Percentage of the ratio of the capacity obtained by discharge to the rated capacity . The greater the numerical , It shows that the stronger its charge holding capacity , The smaller the self discharge rate . Generally, the charge retention capacity of lithium-ion batteries is  85% above .
   Cycle life ： With the lithium ion battery charging 、 discharge , The battery capacity is reduced to the rated capacity  70% when , The number of charge and discharge times obtained is called cycle life .

3、 ... and 、 The advantages and disadvantages of lithium-ion batteries

3.1  Advantages of lithium ion batteries ：
① Large capacity 、 High working voltage . The capacity is twice that of the same nickel cadmium battery , It is more suitable for long-term communication ; The voltage of the usual single lithium-ion battery is  3.7V, It's nickel cadmium and nickel hydrogen batteries  3  times .
② Strong charge holding capacity , The allowable working temperature range is wide . stay （20±5）℃ Next , In an open circuit 

Type storage 28 Days later , The normal temperature discharge capacity of the battery is greater than the rated capacity 85%. Lithium ion batteries have excellent high and low temperature discharge performance , Can be in -20℃～+55℃ Work , High temperature discharge performance is better than other kinds of electricity
pool .
③ Long cycle life . At present, domestic batteries are discharging continuously 300 Next time , The capacity of the battery is still not less than the rated capacity 80%, Far higher than other types of batteries , It has the economy of long-term use .

④ No environmental pollution . The battery does not contain cadmium 、 lead 、 Harmful substances such as mercury , It is a kind of clean “ green 

color ” Chemical energy .
⑤ No memory effect . It can be recharged repeatedly at any time 、 Discharge use .
⑥ Small volume 、 Light weight 、 Higher than energy . Usually, the specific energy of lithium-ion battery can reach that of nickel cadmium battery 2 More than times , Compared with nickel metal hydride batteries with the same capacity , The volume can be reduced 30%, The weight can be reduced 50%, It is conducive to small and lightweight portable electronic devices .

3.2 Disadvantages of lithium ion batteries ：
① The internal impedance of lithium-ion battery is high . Because the electrolyte of lithium-ion battery is organic solvent , Its conductivity is higher than that of nickel cadmium battery 、 The aqueous electrolyte of Ni MH battery is much lower , therefore , The internal impedance of lithium-ion battery is higher than that of nickel cadmium 、 Nickel metal hydride battery is about 11 times .
② The working voltage changes greatly . Discharge the battery to the rated capacity 80% when , The voltage change of nickel cadmium battery is very small （ about 20%）, The voltage of lithium-ion battery changes greatly （ about 40%）. For battery powered devices , This is a serious drawback , However, the discharge voltage of lithium-ion battery changes greatly , It is also easy to detect the remaining power of the battery .
③ The high cost , Mainly cathode materials LiCoO2 The price of raw materials is high .
④ There must be a special protection circuit , To prevent it from overcharging .
⑤ Poor compatibility with ordinary batteries , Due to the high working voltage , So the general case of ordinary batteries with three , Can be replaced by a lithium-ion battery .

Four 、 How lithium ion batteries work

   What we usually call lithium-ion battery is the combination of lithium-ion battery cell and protective circuit , The working principles of lithium-ion cell and protection circuit are explained below ：


  4.1  Working principle of lithium ion battery cell ：
   Lithium ion cell is a device that converts chemical energy into electrical energy , Its cathode is generally lithium intercalation 

matter , Such as LiCoO2、LiNiO2、LiMn2O4 etc. , Lithium is used as negative electrode - Carbon interlayer compound LixC6, Electrolyte is dissolved lithium salt （ Such as LiPF6、LiAsF6、LiClO4 etc. ） Organic solvents . The main solvent is vinyl carbonate （EC）、 Propylene carbonate （PC）、 Dimethyl carbonate （DMC） And chlorocarbonate （ClMC） etc. . During the charging process ,Li+ Back and forth between the two batteries , It's figuratively called “ Rocking chair batteries ”（rocking chair batteries, Abbreviation for RCB）.

   The total chemical reaction is ：
  LiMxOy+nC Li1-xMxOy+ LixCn


   Lithium ion secondary cell is actually a lithium ion concentration cell , When charging ,Li+ Come out of the positive pole , It is embedded into the negative electrode through electrolyte , The negative electrode is in a lithium rich state , The positive electrode is in a lithium poor state , At the same time, the compensation charge of electrons is supplied to the carbon cathode from the external circuit , To ensure the balance of charge . When discharging, the opposite is true ,Li+ Come out of the negative pole , It is embedded into the cathode material through electrolyte , The positive electrode is in a lithium rich state . Take the lithium ion battery with lithium cobalt oxide as the positive pole as an example , When charging , Lithium ion from  LiCoO2  Out of the cell , The ions in it  Co3+ Oxidize to  Co4+; When discharging , Lithium ions are embedded  LiCoO2  In crystal cell , Among them 

Co4+ become Co3+.

  4.2  Working principle of protection circuit ： 

    Due to the chemical properties of lithium ion batteries , In normal use , Its internal chemical reaction is the mutual conversion of electric energy and chemical energy , But under certain conditions , If it is overcharged 、 Over discharge and over current will cause side reactions inside the battery , After the side effect intensifies , It will seriously affect the performance and service life of the battery , And may produce a large amount of gas , The internal pressure of the battery increases rapidly and then explodes, resulting in safety problems , Therefore, all lithium-ion batteries need a protective circuit , For charging the battery 、 Effective monitoring of discharge state , And turn off the charging under certain conditions 、 Discharge the circuit to prevent damage to the battery .
   The following figure shows the schematic diagram of a typical lithium ion battery protection circuit ：

picture

  As shown in the figure above , This protection is routed back to two  MOSFET（V1、V2） And a control  IC（U1） Add some resistance and capacitance elements . control  IC  Be responsible for monitoring battery voltage and loop current , And control two MOSFET  Grid of ;MOSFET  It acts as a switch in the circuit , It controls the on and off of the charging circuit and the discharging circuit respectively . The circuit has overcharge protection 、 Over discharge protection 、 Overcurrent protection and short circuit protection function , Its working principle is analyzed as follows ：





 a. The normal state ：
  Under normal conditions , In circuit  U1  Of “CO” And “DO” Both pins output high level , Two  MOSFET Are on , The battery can be charged and discharged freely . In order to effectively use the discharge current or charging current ,MOSFET  Power tubes with low on resistance are used .
  The diode in the figure is  V1  and  V2  Parasitic diode , Their existence enables the system to charge the battery in the over discharge state , It can discharge the load in the overcharge state .


 b. Overcharge protection ：
  The charging mode required for lithium-ion batteries is constant current / Constant pressure mode , At the initial stage of charging, it is constant current charging , along with 

With the continuous charging process , The battery voltage will rise to 4.2V, Then switch to constant voltage charging , Until the current gets smaller and smaller .
The battery is charging , If the charger circuit loses control , It will make the battery voltage exceed 4.2V Then continue to charge with constant current , At this time, the battery voltage will continue to rise , When the battery voltage is charged to more than 4.3V when , The chemical side reactions of the battery will intensify , May cause battery damage or safety problems .
In batteries with protective circuits , When control IC It is detected that the battery voltage reaches 4.28V（ This value is controlled by IC decision , Different IC There are different values ） when , Its “CO” The pin will change from high level to low level , send V2 From on to off , Thus cutting off the charging circuit , The charger can no longer charge the battery , Play the role of overcharge protection . And now because of V2 The presence of the built-in bulk diode , The battery can discharge the external load through the diode . Under control IC The battery voltage is detected to exceed 4.28V To send off V2 Between the signals , There is still a delay （ The specific delay time is controlled by IC decision , Different IC There are different values ）, To avoid misjudgment caused by interference .

c. Over discharge protection ：
   The battery is discharging to the external load , Its voltage will gradually decrease with the discharge process , When the battery voltage drops  2.7V  when , Its capacity has been completely discharged , At this time, if the battery continues to discharge the load , It will cause permanent damage to the battery .

   During battery discharge , When control  IC  The battery voltage is detected to be lower than  3.0V（ This value is controlled by  IC

decision , Different IC There are different values ） when , Its “DO” The pin will change from high level to low level , send
V1 From on to off , Thus, the discharge circuit is cut off , The battery can no longer discharge the load , Play the role of over discharge protection . And now because of V1 The presence of the built-in bulk diode , The charger can charge the battery through the diode .
Because the battery voltage cannot be reduced under the over discharge protection state , Therefore, the consumption current of the protection circuit is required to be very small , At this point, control IC It will enter the low-power state or sleep state , The power consumption of the whole protection circuit will be minimal （ The specific current consumption value is controlled by IC decision , Different IC There are different values ）.
Under control IC The battery voltage is detected to be lower than 3.0V To send off V1 Between the signals , There is also a time delay （ The specific delay time is controlled by IC decision , Different IC There are different values ）, To avoid misjudgment caused by interference .

 d. Over current protection 

  Due to the chemical properties of lithium ion batteries , The battery manufacturer stipulates that the maximum discharge current shall not exceed 1.5C（C= Battery capacity / Hours ）, When the battery exceeds  1.5C  When the current discharges , It will cause permanent damage to the battery or safety problems .
  During the normal discharge of the battery to the load , The discharge current is passing through the series  2  individual  MOSFET  when , because  MOSFET  On impedance of , A voltage will be generated at both ends , This voltage value  U=I×RDS×2,RDS  For a single  MOSFET  On impedance of , control  IC  Upper “VM” The pin detects the voltage value , If the load is abnormal for some reason , Increase the loop current , When the loop current is large enough to  U＞0.1V（ This value is controlled by  IC  decision , Different  IC  There are different values ） when , Its “DO” The pin will change from high level to low level , send  V1  From on to off , Thus, the discharge circuit is cut off , Make the current in the circuit zero , Play the role of overcurrent protection .
   Under control  IC  Detect the occurrence of overcurrent and send out shutdown  V1  Between the signals , There is also a time delay （ The specific delay time is controlled by  IC  decision , Different  IC  There are different values ）, To avoid misjudgment caused by interference .

  According to the above control process , The magnitude of its overcurrent detection value depends not only on the control  IC  Control value of , It also depends on  MOSFET  On impedance of , When  MOSFET  The greater the conduction impedance , For the same control  IC, The smaller the overcurrent protection value .



 e. Short circuit protection ：

  The battery is discharging the load , If the circuit current is large enough to  U＞0.9V（ This value is controlled by  IC  decision , Different  IC  There are different values ） when , control  IC  Then it is judged as load short circuit , Its “DO” The pin will quickly change from high voltage to zero voltage , send  V1  From on to off , So as to cut off the discharge circuit , Play the role of short-circuit protection . The delay time of short-circuit protection is very short , Usually less than  7  Microsecond . Its working principle is similar to overcurrent protection , Just different judgment methods , The protection delay time is also different .

The following table is a simple summary of the above principles ：

picture

5、 ... and 、 The development direction of lithium ion batteries

   Lithium ion battery as a typical representative of new energy , It has obvious advantages ,

But at the same time, there are some shortcomings that need to be improved . In recent years , Positive and negative active materials in lithium ion batteries 、 The research, development and application of functional electrolyte are quite active in the world , And has made great progress . in general , At present, the development of lithium-ion battery industry shows two trends ： One 、 Polymer lithium ion batteries （ That is, the research and application of new electrolyte ）; Two 、 New positive and negative active substances .

  One 、 Polymer lithium ion batteries ： 

  The so-called polymer lithium ion battery , It is at the positive pole 、 Among the three main structures of cathode and electrolyte , At least one or more batteries made of polymer materials . At present, polymer lithium-ion batteries in the whole industry have only changed the electrolyte , That is, the difference between polymer lithium-ion battery and liquid lithium-ion battery , Is to use polymer electrolyte （ Solid or gelatinous ）. There is no polymer lithium-ion battery that really uses solid electrolyte in the industry , Japan  Sony  company （ Polymer lithium-ion battery industry ranks first ） Gel electrolyte is used .
  Polymer lithium-ion batteries have many advantages ： light 、 thin 、 Large capacity 、 Good safety performance 、 Shape can be arbitrary , therefore , The development of polymer lithium-ion batteries is relatively rapid , Especially in the field of high-capacity batteries , More reflect its advantages  .



  Two 、 New positive and negative active substances ：
  Because the packaging of polymer lithium-ion battery needs to occupy a relatively large size space , At present, the price of polymer lithium-ion batteries is on the high side , therefore , In small capacity batteries （≤1500mAh） field , Liquid lithium-ion batteries still dominate .
  Research direction of liquid lithium ion battery , The key is to keep the volume unchanged , How to improve the capacity of the battery . At present, the whole industry is studying new positive and negative active substances .
  Add a certain amount to the positive active material lithium cobalt oxide  NCA（Lithium Nickel Cobalt AluminumOxide, Lithium nickel cobalt aluminum oxide ） Our products have begun mass production , The lithium ion battery using this active material is better than the lithium ion battery using only lithium cobalt oxide active material , Capacity can be increased  3% above .
  With  NCM（Lithium Nickel Cobalt Manganese Oxide, Lithium nickel cobalt manganese oxide ） Lithium ion battery as positive active material , With lithium cobaltate +NCM  Lithium ion batteries as active substances , Mass production will also begin soon . Tests show that , The former is compared with the current lithium-ion battery , Capacity can be increased  6% above ; The capacity of the latter can be increased  10% above .
   in addition , The research of negative active material is also in progress , With  C+SiO  As a negative active material, lithium-ion batteries will also be put into mass production , Change the situation that the cathode of lithium-ion battery adopts a single carbon material . It is worth mentioning that , With  NCM  As a positive active material 、 With  C+SiO  Lithium ion battery system as negative active material , Its minimum discharge voltage can be  2.5V（ The current lithium-ion battery is discharged to  3.0V  After that, there was basically no electricity ）, In this way, the standby time can be relatively prolonged （ This is also supported by electronic components that can work at low voltage , That is to say  2.5V  When , The whole system of electrical appliances such as mobile phones can be turned on and used normally , At present, the research of electronic components working at low voltage is also a hot direction , Because the voltage is low , The power consumption of components is also relatively reduced ）.