Lithium Polymer Battery Technology:
An Introduction

by Frank Siegert (Graduate Engineer)
June 2015

Originally translated into English by John Julian
He modified Frank Siegert's .pdf file with English annotations

HTML version and American English sentences and/or clarifications by Ken Myers
February 2016

Forward by Ken Myers
The article is somewhat technical but quite "readable".
The sections about Cell Failures and Their Causes, Signs of Damage and How to Provide Long Life to Cells/Packs are found at the end of the article and should be noted carefully. The reader may wish to start with those three sections and then return to the beginning to see how and WHY the statements were formulated.
Read the last three sections now

Special thanks to Dave Stacer, friend and flying buddy, for his assistance in proofing and formatting this document.

Cell Exterior

The illustration shows the exterior of a Lithium Polymer Pouch (LiPo) cell (Zelle). The Cathode (Kathode) is said to accept electrons and the Anode to give up, or lose, electrons. The little blue dots in the lower right corner of the illustration represent electrons.
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Discharging

Movement of electrons

An external view of a cell, during discharge, shows that the electrons (the little blue dots) are said to be moving from the Anode to the Cathode.
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Movement of Li-ions

Inside the cell, during discharge, the Lithium Ions are moving from the Anode to the Cathode.
Electrons flow 'outside' the cell and Lithium Ions inside the cell.
For Electrons, the cell is a non-conductor.
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What Flows in the Cell?
Periodic Table

The Periodic Table shows that Lithium, after hydrogen and helium, is the lightest element. Lithium is an alkali metal with three protons in the nucleus and three electrons in the outer shell.
A Lithium-ion has three protons in the nucleus, but only TWO electrons in the outer shell. That makes it a positively charged ion.

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A Discharged Cell

Empty Cell


The cell strives to reach the state of "lowest energy". This is (theoretically) achieved when the cathode is replenished with lithium.
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Charging
Charging a Cell

During charging, positively charged Lithium ions move from the Cathode to the Anode, which accepts them.
Also, during charging, electrons flow from the cathode to the anode.
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The Cell's Structure

Cell Structure

The cell has a positive (+) Aluminum (Al) terminal and negative (-) Copper (Cu) terminal.
The Cathode consists of Lithium cobalt oxide, Lithium manganese oxide, or Lithium iron phosphate, among others.
The Cathode carrier is made of aluminum.
Typical thicknesses are shown in the diagram for the cathode, separator and anode.
The Anode consists of Carbon (graphite) and the carrier is made of copper or (less often) nickel.

At 1 amp of current for every second there flows 6.2 x 1018 positive Lithium Ions through the cell.

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Voltage Curve
Voltage Curve

The voltage curve is independent for each side of the cell. SOC means state of charge and is represented as a percentage of a full charge.
Halbzelle means half of the cell.
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Voltage Curve Theory Graph

The graph illustrates the voltage curve theory. There are lines for the Cathode (Half-cell 1), Total cell (Gesamte Zelle) = (Half-cell 1 and Half-cell 2) and Anode (Half-cell 2).

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Voltage Curve Theory Measured

Voltage, in the unloaded state, is a measure of the charge status.
With the graph shown above flipped horizontally (shown by the small graph below the larger graph), the smaller graph is changed to a graph showing the state of charge (SOC). The curve then fits the pervious graph showing the voltage curve.
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The Voltage Curve and It's Relationship to the State of Charge (SOC)

Voltage/SOC


Damage can be done to the cell if it is discharged to too low of a voltage or charged to too high of a voltage outside the usable area.
Nutzbarer Bereich means the usable area.
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The Voltage Curve and It's Relationship to the Depth of Discharge (DOD)

Voltage Curve/DOD


The utilization rate of the maximum capacity available.
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Aging Cycles

Depth of Discharge Aging

Aging Cycles


The maximum utilization of the cell yields the minimum lifetime.
The above figures are for about a 1C discharge rate.
High-current discharges lead to a greatly reduced cycle life by accumulation of damage in the cell.
The Maximum current drain yields the minimum lifetime.
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Aging During Storage

Aging at Storage


00C = 320F: 250C = 770F: 400C = 1040F: 600C = 1400F
Deterioration, due to aging, depends on the storage voltage (the higher the voltage the faster the aging) and storage temperature (the higher the temperature the faster the aging).

The total life depends on cycle aging caused by damage in the cell by charging and discharging and storage-aging causing damage in the cell.
agingtotal = agingcycle + agingstorage

The effects of aging are a decrease in capacity, increase in internal resistance, a decrease in current capability, and a decrease in the maximum possible charging current.
Aging occurs nonlinearly. After a certain degree of cell damage, it occurs more quickly (see section "Cell Chemistry" for details)

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The Mechanical Construction of Cells

A Pouch Cell


A Pouch Cell is layered or wrapped in a plastic 'bag' and filled with electrolyte.
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Cell unfolded

The physical construction consists of thin layers, wrapped, folded or 'stacked' for maximum surface area.
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Current and Ion Flow
Currrent and Ion Flow

When charging, the electrons flow from the Cathode to the Anode and the positive Lithium ions move from the Cathode active material through the separator to the Anode active material.
It is important to note that the current flow in the cell is not equally distributed, but focuses near the terminals.
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Heat Development

The heat development of the cell during charging and discharge is composed of two elements:
Irreversible heat
This is due to the ohmic resistance and electrochemical processes during overcharging and/or over discharging.
Reversible heat
This is caused by the electrochemical processes during the normal charge and discharge cycle.

For high-current applications, the irreversible heat generation dominates, especially during discharge.
For a long cycle life, it is important to operate the cell in its optimum working range. Too low of a temperature causes damage, as well as too high of a temperature.

Heat in Cell
Heat in cell, source

Entladung means discharge.
Irreversible heat generation in the cell at a high current discharge is not equally distributed, but localized near the terminals, as shown in the illustration.
Because of this unequal heating, cells do not age uniformly. There is a higher load area.
Also, cells in a pack do not age uniformly. Internal cells cannot dissipate their heat as well.
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Thermal Runaway
Thermal Runaway Diagram

The illustration shows the temperature versus the cell voltage.
The green box is the "Application Window", where the cell is best used.
The wording on the left of the box says, "Formation of copper (Cu) deposits and Cu dendrites causes Cathode disintegration."
The rest of the wording - top to bottom:
"Bursting of the cell and outgassing, if necessary. Fire."
"Disintegration of the Cathode and volatiles released."
"Decomposition of the electrolyte"
THERMAL RUNAWAY
"Separator film melts."
"Disintegration of either layer on the Anode."
"Destruction of the cathode through extreme delithiation"
"Lithium deposition on the Anode at high currents"
"Lithium deposition on the Anode"
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Cell Chemistry

Positive electrode (cathode): Lithiated form of a transition metal (eg, Lithium Cobalt (III) oxide LiCoO2, Lithium Nickel Oxide LiNiO2 or Lithium Manganese oxide LiMn2O4) and mixed forms.
Negative electrode (anode): Carbon (C), usually graphite (C6)
Electrolyte: Lithium salts (LiPF6, LiBF4, LiClO4) and organic solvents (Ethylene Carbonate, Dimethyl Carbonate, etc.)
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Chemical Reactions During Charging

Charging Diagram


The diagram illustrates the chemical reactions during charging. It illustrates the reactions at the Cathode, Anode and Overall (Allgemein).
INNEN: Ionentransport is INTERIOR: ion transport
AUSSEN: el. Strom is EXTERIOR: electric current
The Cathode loses lithium during charging, the anode is filled with lithium.
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Intercalation
Intercalation (from the Latin intercalare = insert), in the chemical sense, is defined as the incorporation of molecules or ions, but rarely atoms, in chemical compounds, which do not substantially alter their structure during the storage process.

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Cathode, ion, Anode Diagram

The diagram once again illustrates the makeup of the cell.

The cell performance depends on:
Current:
The Ion transport capacity
"How many ions can flow per unit of time?"

The more ions that can flow per unit of time yields a High Current Cell.
Limited by:
Surface
Interfaces (Transitions / Layers) yields Terminals
Temperature

Capacity:
Ion release and ion absorption capacity
"How many ions, that the cathode emits during charging, can fit in the anode?"

Yields a high capacity cell

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Anode Chemistry
Anode Chemistry Diagram

The graphite anode consists of layers. Ions can only penetrate "from the side".
This yields a charging rate that is generally lower than the discharge rate.
(inwards is worse than outwards)
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Photo of nanotechnology

The photo demonstrates "Nanotechnology". When using nanotechnology, the graphite layers, with finer ("nanoporous") structure, yields a larger active surface.
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The Importance of Transition Zones or Interfaces
Transition Zones

There are four transition zones shown in the diagram. The most relevant are Zone 2, between the cathode and electrolyte and Zone 3 between the electrolyte and anode. The latter, in particular, is extremely important for the function of the cell.
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Anode Cell Chemistry
Anode Chemistry

The solid electrolyte interphase (SEI) layer protects the graphite anode. The graphite anode would be destroyed by reactions with the electrolyte without it. To stop this process, a protective coating has to be applied on the carbon. This layer is called the solid electrolyte interphase (SEI) layer.
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SEI Layer photo

The SEI layer is composed of the electrolyte and the anode in the initial formation (first charge during the manufacturing process). Lithium ions must penetrate it during charging and discharging.

The SEI-Layer provides protection and stabilization of the anode. Without an SEI layer there is no cycle stability.
Unfortunately the SEI layer results in an increase in internal resistance. The thicker the layer, the higher the resistance.
The SEI Layer also causes a loss of free Lithium.
The thicker the layer, the less remaining capacity there is because of decreased lithium transportability.
The thicker the layer, the more sensitive it is to plating deposits.

The SEI layer is basically constructed in the preparation of the cell. Any cell which is 'new' has already experienced at least one preparation cycle.
The SEI layer grows during the life of the cell. The more degradation products that are caused in the cell, the thicker it is. High pressure and temperature accelerate this degradation.

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Anode Plating

The deposition of metallic lithium ions at the anode, when charging, is called plating.
Dichtegradient means density gradient.

Plating occurs when 'the anode wants more Li+ ions in' than can be accommodated with the transportation capacity available.
This happens when charging, not during discharge.
When the cell reaches maximum capacity, overload can occur to the already fully replenished anode (greater than 4.2V in normal cells).
The transport capacity can be reached by using too high of a charging current, charging at too low of a temperature and the state of the cell's aging, caused by deposits of the oxidation products of the electrolyte and SEI Layer thickness.

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Photo of dendrites

The Lithium plating does not form in clean 'layers' but in "towers" called dendrites.
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Diagram of Flaws

The plating is also caused by "flaws".
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Anode bigger than Cathode

Also, in production, the anode is always designed to be somewhat larger than the cathode, otherwise, at the edge, zones plating would occur.
This excess material on the Anode is referred to as "Overhang".
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A micro short

Plating causes Lithium loss and Lithium dendrite formation if the separator is bridged. This condition is called a "Micro Short".
wenige 10 μm means a few 10 microns
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A macro short

"Micro Shorts" can become "Macro Shorts" if there is an extreme local temperature increase, the separator is damaged, or there is local collapse of the SEI layer causing an exothermic reaction. The resulting defect leads to further plating.
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More on Thermal Runaway
Runaway Temperature Graph

Thermal Runaway occurs from about 1500C (3020F) when the "Self-heating rate" is greater than 100C (500F) per minute.
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Cell heating diagram

Thermal runaway starts around 1500C (3020F). The heat build-up to the 'Onset' is primarily based on resistive heating by the "Short", to about 2500C (4820F), then it becomes a significant exothermic decomposition processes.
If the runaway temperature is reached locally, and the affected area is sufficiently large, it spreads very quickly through the entire cell.
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A void

If there is not sufficient heat generation, then the result is only a void. The self-limitation occurs in part because the separator expands when heated and thereby locally lowers its ion conductivity. As long as the separator does not melt, the short circuit will be limited. However, this fault can possibly act as a trigger for further plating.

The temperature rise to 'Onset' can also occur slowly (minutes to hours). Once this is achieved, however, the heating occurs faster locally.
The warming 'Onset' gets its energy from the charging of the cell. In a cool storage environment, a higher heat generation is necessary. This also assumes the necessary size of the triggering 'Shorts' and Bigger 'Shorts' are rare and reach a certain size only by mechanical damage.
From outside, these processes in the cell are not easy to detect.
For a small cell capacity this damage can be detected from a slightly elevated self-discharge. For larger capacities, however, this indication is weaker. In such cells, at less than 3.3V, the problematic cells can only be detected in test runs by repeated slow charge amd discharge in the low cell voltage range. On the basis of small, impulsive changes the charging current can be detected.
Micro-shorts cannot easily be ruled out, so you have to minimize the likelihood that they might trigger runaways.

A cell must have sufficient energy stored for a runaway. The higher the SOC, the more likely it is.
High ambient temperature favours a runaway.
Advice: It is best to keep the pack cool, 200C (680F) or less, and store at a low state of charge (SOC).

Factors for the formation of plating include high charging current, low temperatures (while discharging), old or overloaded cells with a thick SEI layer, flaws from manufacturing defects or damage, or too high of a discharge current. (See explanation of the Cathode.)
Advice: Never cold discharge cells at a high C-rate load.

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Chemistry of the Cathode
Cathode Chemistry

The Cathode consists of Lithium Metal Oxides, such as LiCoO2.
The diagram shows Oxygen (Sauerstoff), Cobalt, and Lithium.

When charging, the positive lithium ions move from the cathode to the anode.
Upon discharge, the cathode accepts positive lithium ions.
As with the anode, there is a transition zone ("interface") to the electrolyte.
In contrast to the anode, there is no "SEI layer", but instead, reaction products diffuse into the electrolyte.

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Cathode Stress

At high discharge rates, the cathode is under mechanical stress.
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Cathode Stress

High discharge rates, especially at low temperatures, leads to cracks and fractures in the Cathode.
Advice: Never overload cold cells!
COLD CELLS is NOT defined in the document. It is 'hinted' at in the section "Cell Failures and Their Causes/Temperature Related Failures" where it says, "If the cell temperature is too low, less than 150C (590F)...
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Reaction Process

Temperatures that are too high, even locally, and/or cell voltage greater than 4.1V causes a reaction process (oxidation) of the electrolyte at the cathode layer.
The reaction products are deposited primarily on the SEI layer at the anode.
This process causes an increase in the internal resistance and the loss of capacity.
These reactions also produce gases (mostly CO2). The gasses are the main reason for irreversible "puffing" after an overload or storage at a fully charged state (especially at high temperatures).
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Capacity Curve

Accumulation of the oxidation products on the SEI Layer is the main reason for the 'kink' in the loss of capacity curve shown in the graph above.
This goes hand in hand with an increase in the associated internal resistance.
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Initial capacity

Initially there is minimal effect on the capacity as the SEI surface is sufficient.
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Increasing

Increasing the effect, the SEI surface is still just enough.
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Increasing the effect more

At the start of accelerated aging, the internal resistance has increased significantly, plating sets in, and the lithium loss becomes relevant.
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Dead cell

Once the plating is dominant, the Lithium loss is high. The cell clearly loses capacity, while strongly increasing its internal resistance.
Advice: Do not overload/overburden the cells.
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Excessive Temperatures

Excessive temperatures lead to the release of free "metals" at the cathode. This causes "poisoning" of the SEI layer
This is a problem mainly in LiFePO4 cells as it triggers iron deposition at temperatures greater than 450C (1130F).
Advice: Do not store cells in a warm environment!
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Cell Damage

Excessive Temperatures


Cell damage accumulates over the period of use. A cell does not forget mistreatment!
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Chemistry of the Electrolyte
Electrolyte Chemistry

The electrolyte can be in the form of a liquid or gel. There are three basic components of the electrolyte; "Electrolyte salt" (LiPo), LiPF6, organic solvents, including EC, PC, DMC, EMC, and additives.
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Solvents

The mixing ratio of the organic solvent determines whether, at room temperature, it is more liquid or gel-like.
EC is Ethylene Carbonate
DMC is Dimethyl Carbonate
EMC is Ethyl Methyl Carbonate
Simplified: The higher the EC share of the mixing ratio the more solid, and thermally stable at high temperatures, the electrolyte is.
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Role of EC

Ethylene Carbonate (EC) is always included in the mixture. It is necessary for the formation of the SEI layer on the Anode. It builds the SEI layer on the Anode.
EC yields the SEI Layer that is formed in the first cycle.
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Electrolyte Mix Ratio

The mixing ratio determines the temperature and voltage capability.
Using EC and DMC the electrolyte is unstable from about 4.25V.
Using EC, DMC and diethyl (DEC) in a 3:3:4 ratio, the electrolyte is stable to 4.5V.
The EC proportion has declined in recent years (From about 50% to about 25%).
High Voltage (HV) cells have smaller proportions of EC in the electrolyte and are therefore slightly less temperature stable.
The graph shows the changes between the year 2000 electrolyte and 2014 electrolyte and compares them to an HV electrolytic.
There is a smaller EC-share from the year 2000 through the HV type.
That has caused a shift in the usable temperature window.
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Electrolyte Composition Table

Depending on the electrolyte composition, the formation, after the first charge, is not yet fully completed.
Advice: Gentle treatment of the cell in the first cycles may therefore be useful.
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Additives

Additives are used to:
Improve the SEI formation
Protect the cathode and electrolyte (Oxidation inhibition and Li Retention)
Provide LiPF6 temperature stabilization
Provide overload protection ("Redox Shuttle")
Provide fire prevention ("Fire-Retardant")
Provide a wetting agent (Manufacturing, wetting of the electrode)
Provide corrosion protection for the Aluminium
In 2014 there were about 3 to 5 additives used in each cell.

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Example of an HV additive

HV cells have improved cathode stability by using the additive Li2CO3.
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Cell Failures and Their Causes

Failures Caused by Voltage Errors

If the voltage is too high, greater than 4.25V, the Cathode has stress caused by extreme delithiation, the Anode suffers from lithium plating yielding Lithium dendrite formation and the Electrolyte decays due to the oxidation processes at the cathode.

If the voltage is too low, less than 2V, the Cathode has stress due to high delithiation and Oxygen is released, the Copper carrier dissolves releasing oxygen and producing copper (Cu) dendrites on the Anode.

Temperature Related Failures

If the temperature is too high, greater than 600C (1500F) (greater than 550C (1310F) with newer type cells) the Cathode releases oxygen and the Anode has a SEI layer breakdown.
There is an increased risk of thermal runaways!
With too high of a temperature, the Electrolyte decays.

If the cell temperature is too low, less than 15-200C (59-680F) at a high C-rate, the Cathode is stressed from too high of a lithium-ion flow and breakage and cracks occur during discharge.
If the temperature is too low, Lithium-Plating occurs during the charge on the Anode.

Discharge Rate and Charge Rate Failures

If the discharge rate is too high, the Cathode suffers from oxidation of electrolyte (especially at high temp) and there is stress caused by the high lithium-ion flow. That stress causes breaks and cracks in the Cathode, especially at low temperatures.
If the discharge rate is too high, the Anode accumulates products of degradation from the electrolyte.

If the charge rate is too high, and in particular at low temperature, the Anode suffers from plating.

Cell Puffing Failures

"Puffed" cells are divided into two different types:

"Reversible" puffing occurs in a high-current discharge. The cell reforms after cooling back down. This is basic puffing caused by the evaporative components of the solvent in the electrolyte. It is a sign of a current overload, but cell can still be used.

"Irreversible" puffing does not reverse after cooling. Its basic cause is the decay processes of the electrolyte forming CO2 and other gases. It is an indication of defects in the cell. The consequence is a loss of capacity and increased internal resistance. If a deep discharge occurred, immediately discard cell. There is probable damage to the cathode, which was accompanied by oxygen release.

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Signs of Damage

Damaged cells smell 'sweet' or 'fruity' if electrolyte leaks. Such cells must be disposed of immediately. Only hold them with protective gloves, as the conductive salts in the electrolyte are poisonous and corrosive. Some additives are dangerous as well.

Conducting salts ("LiPo") LiPF6:
R34: Causes burns.
R24: Toxic in contact with skin.
R22: Harmful if swallowed.

Conducting salts ("LiFePo") LiBF4:
R20/21/22: Harmful by inhalation, and with skin contact.
R31: Contact with acids liberates toxic gas.
R34: Causes burns.
R36/37/38: Irritating to eyes, respiratory system and skin.
R23/24/25: Toxic by inhalation, in contact with skin.
R11: Highly flammable

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How to Provide Long Life to Cells/Packs

Store cool, dry and empty (about 3.7V per cell) at or less than 200C (680F)
Bring the pack up to operating temperature before charging and discharging.
Never overcharge greater than 4.2V per cell, or great than 4.25V or 4.35V per cell for HV cells.
Never over-discharge to less than 3.0V per cell. If an over-discharge occurs, immediately recharge.
The cell's operating temperature should be greater than 250C (770F), ideally between 300C (860F) and 400C (1040F)
Keep the pack at working temperature only as long as absolutely necessary
Do not operate them if overly warm, greater than 550C (1310F) to 600C (1400F).

Even longer life:
Do not charge to 4.2V (cycle life is doubled if charged to just 4.1V *)
Do not fully discharge, use shallow cycles (less than 70% DOD doubles lifespan *)
Use low loads and do not load quickly (especially at low temperatures)
* especially for low loads

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Return to Beginning of Article

Credits
Created by Dipl.-Ing. Frank Siegert frank@wizards.de
June 2015.
This work is licensed under a Creative Commons Attribution - Non-Commercial - Share Alike conditions 4.0 International License//.

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Danksagung:
Ich bedanke mich bei Gerd Giese für sein hilfreichen Tips und Korrekturlesen sowie Ludwig Retzbach für seine hilfreichen Hinweise. Auch gilt mein Dank RC-Network.de für die Bereitstellung einer Platform für Diskussionen über Lithium-Akku Technologie.