Configsource path using wrongly

I'm trying to use a config file by directing there from App.config. I have created a folder named Config inside my solution and created a new config file named Environment.config.

My App.Config looks as following:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <configSections>
    <section name="entityFramework" type="System.Data.Entity.Internal.ConfigFile.EntityFrameworkSection, EntityFramework, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" requirePermission="false" />
    <!-- For more information on Entity Framework configuration, visit http://go.microsoft.com/fwlink/?LinkID=237468 -->
  </configSections>
  <entityFramework>
    <defaultConnectionFactory type="System.Data.Entity.Infrastructure.SqlConnectionFactory, EntityFramework" />
    <providers>
      <provider invariantName="System.Data.SqlClient" type="System.Data.Entity.SqlServer.SqlProviderServices, EntityFramework.SqlServer" />
    </providers>
  </entityFramework>
<startup><supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" /></startup>
  <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <dependentAssembly>
        <assemblyIdentity name="Newtonsoft.Json" publicKeyToken="30ad4fe6b2a6aeed" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-9.0.0.0" newVersion="9.0.0.0" />
      </dependentAssembly>
    </assemblyBinding>
  </runtime>
  <appSettings configSource="Config/Environment.config"/>
</configuration>

and the Environment.config looks as the following:

  <appSettings>
    <add key="URL" value="http://foo.aspx"/>
  </appSettings>   

I'm getting an error which says:

Result Message:
OneTimeSetUp: System.Configuration.ConfigurationErrorsException : Configuration system failed to initialize ----> System.Configuration.ConfigurationErrorsException : The configSource attribute must be a relative physical path, so the '/' character is not allowed. (D:\tfs\QA - Automation\Projects\ReportAppeal\ReportAppeal\bin\Debug\ReportAppeal.dll.config line 22)

I have tried to switch from "/" to "\" but got a different error.

Result Message:
System.Configuration.ConfigurationErrorsException : Unable to open configSource file 'Config\Environment.config'. (D:\tfs\QA - Automation\Projects\ReportAppeal\ReportAppeal\bin\Debug\ReportAppeal.dll.config line 22) TearDown : System.NullReferenceException : Object reference not set to an instance of an object.

I'll probably need to change the way I'm directing the Environment.config file but I'm not sure how.